Unit2019 pad

UNIT 2019

Know how to prepare
surfaces for decoration 2
Surface preparation is very important for producing a high-quality
finish. In this unit we will look at some typical surfaces you may
find yourself working on during your career, along with appropriate
preparation tasks for each surface.

It is important that all surface contaminants such as dirt, oil,
rust and loose or flaking existing coatings are removed. If these
contaminants are not removed, it could affect whether the paint
or paper will adhere (stick) to the surface.

This unit also contains material that supports NVQ unit QCF 332
Prepare surfaces for Painting/Decorating.

This unit also contains material that supports TAP Unit Prepare
surfaces for Painting/Decorating.

This unit covers the following learning outcomes:
Preparing timbers and timber sheet products ready to receive
finishing systems
Preparing metal surfaces ready to receive finishing systems
Preparing trowelled finishes and plasterboard ready to receive
finishing systems
Removing previously applied paint and paper ready to receive
finishing systems
Rectifying surface conditions
Repairing and making good surfaces

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Level 2 NVQ/SVQ Diploma Painting and Decorating 3rd edition

K1. Preparing timbers and timber
sheet products ready to receive
finishing systems
Timber is one of the most commonly used materials in
construction. You will encounter timber in a wide range of
Key terms situations, both internal and external.
Botanical – the classification
of trees based on scientific
study
Applications of timber and timber
Deciduous – the name given
sheet products
Unit 2019 Know how to prepare surfaces for decoration 2

to a type of tree that sheds it Types of timber
leaves every year
Timber is classified as either softwood or hardwood. This can
Evergreen – the name given
sometimes be confusing as not all hardwoods are physically
to a type of tree that keeps its
leaves all year around
hard or softwoods soft. Hardwood and softwood refers to
the botanical differences and not the strength of the timber.
Hardwood trees are deciduous, broad leaved, with an encased
seed. Softwood trees are usually evergreen with needles and
seeds held in cones.

Name Appearance Properties/Description Uses

Redwood Moderately strong for its weight with Used for interior or exterior
(commonly average durability. Quality of finish work and for carcassing and
known as pine) depends on knots and amount of finish joinery
resin. Capable of smooth, clean finish
and can be glued, stained, varnished
and painted.

Whitewood Similar to redwood in strength and Similar uses to redwood
(also known durability. Takes glue, nails and screws
as European well and can produce a good finish.
Spruce)

Western red Not as strong as redwood but has Externally for good-quality timber
cedar naturally occurring oils which prevent buildings, saunas, etc.
insect attack. Doesn’t need treating as
will stand up to severe weather and
turns a silvery colour when exposed.

Table 19.1 Commonly used softwoods

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Oak Very strong with English oak the High-class joinery, panelling, doors,
strongest. Good resistance to exposed roofing, etc.
bending and shearing. Susceptible
to fungi attack and ironwork
should not be used as it will stain
and disfigure.

Beech Hard, close grained and durable Furniture, kitchen utensils, wood
with a fine texture. Capable of a block floors, etc.
good smooth surface. Takes glue,
stains and polish well and can
produce an excellent veneer.

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Mahogany Strong for its weight and High-class joinery, furniture, boat
moderately resistant to decay. building and plywood veneers
Takes glues, finishes, nails and
screws well.

Know how to prepare surfaces for decoration 2
Table 19.2 Commonly used hardwoods


Plywood Made from thin layers of timber Used in floors, walls and roofs
glued together to form boards.
Alternating grain across and
along the sheet gives strength
and stability.

MDF Made from pulped wood mixed Skirting boards and mouldings
with adhesives and pressed
into sheets. Moisture resistant
MDF is available.

Hardboard Manufactured from sugar cane Flooring, furniture and units
pulp mixed with adhesives and
pressed into sheets 3–6 mm thick.
Has a reasonable resistance
to moisture.

Table 19.3 Commonly used timber sheet products

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Uses of timber
Timber has several different uses in the construction industry.
You will be familiar with some of these uses, particularly
structural uses, from Unit 2003, pages 81–85 and page 88.
l Structural – floors, walls and roofs providing stability to
the building. Timber is a core part of the construction or as
decking over a framework.
l First fixing – any work inside a structure before plastering

takes place. Includes studwork, ground lats, stairs, windows
and doors.
l Second fixing – any work inside a structure after plastering

has taken place. Includes all joinery work, such as doors,
architrave, units and ironmongery.
l Decorative – any structure made entirely from timber such as

fire surrounds, mouldings, balustrades, banisters, dado rails,
Safety tip
flooring and decking/fencing.
Knotting solution is highly
flammable and so should not
be exposed to naked flames.
Defects in timbers and timber sheet
You must also make sure you products, and treatments
wear the appropriate PPE when
handling this material.
Timber can be affected by a wide range of defects. If you are not
sure what preparation or decoration is required for a particular
type of timber, you should always seek advice before starting
the task.

Knotting and resin exudation
A knot is a place in the timber where a branch was joined to
the tree. Resin may bleed from knots, staining the paint finish.
Knotting solution seals knots and can be applied to areas stained
with resin, tar and ink. Its main ingredient is shellac, produced by
an insect and melted into thin flakes. Clean and dry the surface
before applying with a brush. It should dry quite quickly, after
which time the surface coating can be applied.
Figure 19.1 Knotting bottle Stain sealing
Shellac is also available coloured (known as pigmented shellac).
Key term Aluminium provides a silver pigment while titanium provides a
Resin – a natural liquid formed white pigment, and these are very effective stain sealers
in wood when it is converted
especially on:
into useable timber. Very
sticky and usually yellowish l stains made by fire, smoke and water
l previously creosoted timber
gold, under heat it emerges
from knots (this is called resin
l animal, smoke and fire odours (smells).
exudation)

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End grain, open grain and cracks Did you know?
Wood grain is the growth rings found inside all tree trunks. When Shellac is not only used in stain
timber is cut in the opposite direction of these rings this is called sealers. It can also be used as a
cutting ‘against the grain’ and it can cause timber to chip or tear. safe coating on foods, such as
fruit and sweets, to give them a
Other grain cuts include:
glossy shine.
l with the grain – cutting the timber is easier and cleaner
l across the grain – cutting across the grain lines but the plane

of the cut is still aligned with them
l end grain – the timber is cut at right angles to the grain, for

example when trimming the ends of planks
End grain often needs to be repaired by the decorator by stopping

Unit 2019
and filling, which should be done after priming. A stopper is stiff Key term
material used to ‘make good’ gaps and holes on surfaces and it Open joints – gaps in timber
dries with the minimum amount of shrinkage. Areas such as open structures
joints and splits in timber need to be stopped and made flush
prior to decoration.
There are several different types of filling methods that you will
need to use.

l Proud filling – overfill and leave a raised amount of material
filler (proud). After drying the filler will reduce or shrink back
leaving it slightly proud. It will need abrading level.
l Back filling – press the material deep into the area then leave

it to dry. Repeat the process until the surface is level. Figure 19.2 Stoppers
l Flush filling – use a filling knife or caulk board to apply the

filler and make the surface flush prior to applying coatings.
Filler used for good open-grained timber
l Plastic woods – made from a mixture of resin and wood flour,
used when applying clear wood finishes. Available in a ready
mixed formula and two-pack formula. Very quick setting Key term
but expensive. Flush – when one surface is
l Two-pack stoppers – hard-wearing and can have fittings level and even with another
screwed into them. Dry very quickly and have little shrinkage. surface
Must be used on bare surfaces as they can cause coatings to
become defective due to the ingredients in the stopper.
l Putty – substance mixed with linseed oil which becomes sticky.

Commonly used on wood for filling holes and to fix glass
into window frames. Less expensive than other stoppers but
becomes brittle with age.
Moisture content
Wet rot is a growth of brown fungus in damp timber. As the
fungus grows, it destroys the wood. The only long-term treatment

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Find out of wet rot is the removal of moisture. Before working on an area
Why should you not put iron affected by wet rot, you must treat it, as outlined here.
screws into timber affected by l Rake out any defective timber. Allow the surface to dry and
wet rot?
flood the timber with a clear wood preservative. Allow it to dry
and spot prime the affected areas with wood primer.
l Fix wood screws (non-ferrous, that is non-iron) into the timber.

l Apply a coat of two-pack polyester filler to the surface and

allow it to dry. The screws will help the filler adhere to
the surface.
l Apply a second coat of filler and allow to dry. Abrade the filler

so it is flush. An acrylic spot filler (a soft putty) may be needed

to fill any minor imperfections.
Glue residue and nail heads
Glue residue is excess glue left on the surface of timber. If dry,
this can be removed with a scraper and then sealed with knotting
solution. If wet, remove with scraper and clean with white spirit or
turps and leave to dry.
Nail heads may be left projecting above the surface. Place a nail
punch squarely onto the nail head, covering the whole surface of
the head, and hammer the nail further into the timber until it is
below the surface. Apply a stopper or filler to the hole.

Surface and physical properties of
timbers and timber sheet products
Tactility and aesthetics are the key characteristics of wood
and timber.
l Tactility – how workable the timber is to create different
structures and fittings. The more workable it is, the more
Figure 19.3 Tactility and potential uses it will have. Tactility also applies to other
aesthetics: the workability and materials, such as plaster.
beauty of timber l Aesthetics – the finishing look of timber when it has been

stained and varnished. The more pleasant the aesthetic look,
the more likely it is to be used decoratively.
Porosity
A porous surface is one that contains tiny holes through which
liquids or gases can pass. In order to prevent porous surfaces,
such as timber, from being penetrated by water or damaged by
frost, a silicone water-repellent layer can be applied to waterproof
the surface. When dry it is completely clear.

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Physical properties
Timber has a number of physical properties. Some of these
properties also apply to other materials you will encounter –
the qualities they give will be essentially the same for each type
of material.

Insulation Ability to trap heat. Wood has low thermal conductivity so it is a natural
insulator. Air pockets in the wood make it a barrier to heat and cold Figure 19.4 Silicone water
Hardness Resistance to wear and tear. Also difficulty to saw or plane the timber repellent used on porous timber
Strength Amount of stress timber can resist before bending. The ‘stiffer’ the wood,
the stronger it will be

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Flexibility Ability to expand and retract without damage

Absorption Ability to absorb shocks and damage

Adhesion Ability to ‘stick’ or fix to a different surface

Table 19.4 Physical properties of timber

Capillary action

This is a process where liquid is drawn up through a small gap
between the surfaces of two materials. In a building this could
allow water to rise up from the surface and into the building.
Timber can be affected by this if its moisture content is above
30 per cent. Capillary action will also cause the timber to rot.
This also applies to plaster and metal areas. These will not rot Remember
but plaster will become weaker and metal will corrode Abrading a new softwood
(see page 111). or hardwood may result in
damage due to scratching or
furring (the lifting of wood
Appropriate abrasives fibres). For this reason, it is best
Abrading a surface means wearing away the top layer by rubbing, to simply dust off the surface
prior to painting.
or creating friction. This is a very important part of surface
preparation. It provides a key for the coating or covering to be
applied and smoothes the surface to give a good-quality finish.
It is important that the correct type of abrading material is used. Key term
Key – roughness on a surface
l Abrading material that is too rough can leave scratches on
provided to aid adhesion
surfaces that show through to the finish.
l Material that is too fine can have a long preparation time

and be ineffective at removing or levelling rough surface
imperfections.
l Cheap and inadequate materials (such as glass paper) can

greatly extend the preparation time as they get blunt and clog
very quickly.

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Abrading materials and equipment
Grades of abrasives
It is important to select the correct grade of abrasive for each job
to achieve the correct finish. The grade is printed on the back of
abrasive paper and relates to the particles of aggregate to every
square 25 mm.
A grade that gives a coarse abrading effect will have large particles
and therefore less of them. Figure 19.5 shows aggregate on a P20
grade dry abrading paper where only 20 particles of aggregate will
Figure 19.5 A small number
fit on to a 25 x 25 mm area.
of large aggregates will give a

coarse abrading effect A grade that gives a fine abrading affect will have lots of small
particles. Figure 19.6 shows aggregate on a P80 grade dry
abrading paper where 80 particles of aggregate will fit on to a
25 x 25 mm area.

Wet and dry abrasives
Harcourt Education
These can beJ6637 Paintingboth wet and dry conditions. A waterproof
used in and
Decorating
adhesive fixes9pt Zurich BT
the abrasive particles to the backing, which means
AW031
that the paper does not lose the particles when it gets wet – in
fact, if wet and dry paper is used dry it tends to clog up.
The aggregates used in wet and dry abrasive paper are often
silicon carbide, but aluminium oxide is becoming increasingly
Figure 19.6 A large number of
popular. Particles of aggregate are closely grouped together and
small aggregates will give a fine
abrading effect
referred to as being ‘closed coated’. Water (sometimes mineral
oil) can be used as a lubricant, preventing paper from becoming
clogged. Wet and dry abrasive is available in grades from P80
(coarse) through to P1200 (very fine).

Remember Advantages Disadvantages
Always choose the correct type
Extremely good for Harcourt Education
high-quality work More expensive than some dry abrasives
and grade of abrading material J6637 Painting and
for the surface and the job. Wide range of grades available
Decorating Unsuitable for bare timber
9pt Zurich BT
AW032
Cleans the surface as it abrades Clogs up easily if used dry
Low dust levels Surface must be dry before decoration
Safety tip
Abrading will create dust Table 19.5 Properties of wet and dry abrasives
particles so ensure adequate
ventilation of the work area and
Dry abrasives
wear appropriate respiratory These use a non-waterproof adhesive to fix the abrasive particles
PPE. Some tools are equipped to the backing paper. The best aggregate to use in this type of
with a dust collection bag. paper is aluminium oxide grit. Glass and garnet are common,
but less effective.

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Aluminium oxide abrasive (production paper) is usually available
‘open coated’, where the particles of aggregate are spaced apart New abrasive
on the backing paper. This reduces the risk of clogging as the
gaps allow waste to escape. Dry powder lubricants can be used on
some types of dry abrasives, breaking away when heat is generated Aluminium
oxide particles
by the abrading process, preventing clogging of the abrasive. wear down and
break away
Dry abrasives are available in grades ranging from P20 (coarse)
through to P320 (very fine).
Remaining
particles are
smaller and
Advantages Disadvantages sharper

When worn, particle edges shear off Aluminium oxide can be expensive
Figure 19.7 How aluminium
revealing smaller but sharper edges compared to other abrasives
oxide breaks down

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(Figure 19.7)

Available in sheet, roll, disc and belt form High dust levels produced
Available in self-adhesive rolls – abrasive
can be torn off and fixed to purpose-made
rubbing blocks

Table 19.6 Properties of dry abrasives
Figure 19.8 Belt sander

Mechanical sanding
Electric tools can greatly reduce time spent preparing surfaces
and increase the surface area covered. Electrical sanders work by
moving an abrasive pad or belt at a fast speed.

Belt, drum and orbital sanders
The heavy duty sanders most commonly used by a decorator are
belt, drum and orbital sanders.
Belt and orbital sanders are hand-held power tools best used for
sanding large, flat items of joinery. A drum sander is self-propelled
and used for stripping floors. A rough grade of abrading material
should first be used to remove surface coating. The surface can
then be brought up to a smooth finish by progressively using finer
and finer abrading material.
Advantages Disadvantages
Figure 19.9 Drum sander
Effective at abrading large areas More expensive than abrasive papers
Faster rate of abrasion than by hand Only suitable for large, flat areas
Can create large amounts of dust

Table 19.7 Properties of belt and orbital sanders

Figure 19.10 Orbital sander

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Did you know? Disc or rotary sanders
Small electric sanders are also Rotary sanding involves the use of rotating discs of abrasive
available with triangular heads material and can be used to prepare small or contoured surfaces.
for use when sanding corners.
Different types of abrasive disc are available:
Abrasive discs can be fitted to
electric drills and angle grinders. l flat discs that require a backing pad
l flap discs made up from flaps of abrasive, which are more

expensive but also more effective
l grinding discs that can be used for removing very heavy, small

areas of rust.
Advantages Disadvantages

Do not burnish the surface Only suited to small areas
Effective at removing isolated patches Not suited to complex surfaces (discs
of rust cannot reach into awkward corners)
Figure 19.11 Disc (or rotary) Relatively low initial cost of equipment
sander – in this instance, an
electric drill fitted with an Table 19.8 Properties of disc or rotary sanders
abrasive disc attachment
If the sander is equipped with a dust collection bag, make sure it
is working and empty it before using the tool. After sanding, the
wood dust collected should be disposed of appropriately. Sanding
dust should not be left in bags indoors as there is a danger of it
catching fire through its own heat.

Appropriate solvent-based and
water-based primer
Priming is the first coat of paint applied to a surface. Primers
protect the substrate and give an even and consistent finish to the
final coat. Universal primers are designed to be used on a range of
surfaces. If the surface preparation or the application and choice
of the primer is incorrect in any way, the durability of the paint
system will be reduced.
l Solvent-based primers – form a waterproof layer to prevent
wet rot in timber surfaces. They do not raise the grain of the
wood when applied, but have a longer application time than
water-based primers as the solvent content prevents them from
drying as quickly as water-based primers.
l Water-based primers – provide a moisture screen to

the surface. Water-based primers do not soak into timber,
meaning they adhere less well than oil-based paints. They are
low in VOCs and odour, quick to dry and more durable on
hardwoods.

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Primer Description

Acrylic primer Water based. Reapplied as an undercoat to speed up
coating process. Mostly used for internal timbers but can
be used externally.

Aluminium wood primer Solvent based. Has aluminium non-leafing particles to
make it more suitable for priming resinous timbers such
as Columbian and Oregon pines. Can be used for both
internal and external timbers.
Wood primer Solvent based. Can be used on softwood and hardwood
internal and external timber surfaces.
Preservative primer Solvent based. Used for external timbers only. Similar to
wood stains and varnishes as it protects the timber

Unit 2019
Table 19.9 Types of primer

Correct preparation process for
rectifying defects in timber
There are different preparation techniques for each type of timber
surface. Follow the techniques described earlier in this unit, as
well as remembering the different qualities of types of timber,

before you begin work.

Bare untreated timber
For basic painting tasks:
l seal any knots in timber using knotting solution
l prime the surface using oil-based wood primer (for external

surfaces) or acrylic primer undercoat (for internal surfaces)
l fill using polyfiller and decorator’s caulk, rub down and dust off

l apply one coat of undercoat, rub down and dust off

l apply another coat of undercoat if necessary then apply one

coat of gloss
Alternatively, for staining or varnishing tasks:
l fill holes in timber with putty or coloured stopper and apply
a base coat
l rub down and dust off

l apply one coat of wood stain or varnish and lightly rub down

and dust off
l apply second coat of wood stain or varnish.

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Previously painted timber
l Rub down using sandpaper. If necessary, fill using polyfiller
or caulk.
l Apply one coat of undercoat, rub down undercoat and dust off.

l Apply one coat of gloss.

Rough cut timber
Rough sawn timber should be dry brushed thoroughly to remove
soil, vegetation and dust.
l Apply one coat of timber preservative or wood stain.
l Apply a second coat of timber preservative or wood stain.

K2. Preparing metal surfaces ready
to receive finishing systems
Metal has a wide range of uses throughout buildings and
structures. It can be used for frames, girders and trusses. Some
Key term
metals, such as aluminium, are also used for decorative reasons.
Element – a substance
There are several different types of metal that you may encounter
that cannot be broken down
into any other substance.
as you work. Metals are either pure or alloys.
For example water is not an l Pure – the metal is made from only one element. Common
element as it can be broken examples of this include gold, silver, lead, copper, aluminium,
down into hydrogen and
zinc, iron and tin.
oxygen. These cannot be
broken down so are elements l Alloy – a mixture of two or more metal elements, used when

a strong, light metal with properties that do not exist in a pure
metal is needed. Aluminium alloy is a common example of this.

Surface and physical properties of
metal types
Metal shares some qualities with timber, including porosity (page
102). The other key qualities of different types of metal are shown
on page 109. Metals will not exhibit all these qualities.

108


Quality Description

Malleability Can be hammered and pressed into different shapes

Conductivity Has the ability to conduct heat and electricity

Elasticity Can regain its shape after being misshapen during use

Hardness Resistant to scratches and cuts during construction

Brittleness Extremely hard but can be broken very easily

Ductility Can be stretched without breaking and turned into a fine wire. Find out
Metals with this quality are used for metal cables. Use the Internet as well
Toughness Can absorb shock and energy without breaking (the opposite as materials lists and

Unit 2019
of brittleness) manufacturers’ information
Tensile strength Tested under extreme conditions (pulling, squashing, twisting and to find out more about
shearing) and can withstand these forces the circumstances where
these qualities of metals are
Heaviness Denseness of the metal in relation to its size particularly desirable.
Strength Tested to withstand heavy loads without breaking

Table 19.10 Key qualities of different types of metals

Types of metal and their applications
Non-ferrous metals
Non-ferrous metals do not contain iron and are not magnetic.
They are usually more resistant to corrosion as they have
non-friable oxide layers, created by the atmosphere. These should
be dry and free from grease prior to painting. Previously painted
non-ferrous metals need to be abraded and any corrosion deposits
found should be scraped back to a firm edge where any flaking
paint is evident.

Ferrous metals
Ferrous metals contain iron and may have small amounts of other
metals and elements added to them to give them the properties
they need. Most ferrous metals are magnetic. These surfaces
are prone to rusting and will need to be cleared of all rust prior
to painting. Depending upon the extent of the rust, it can be
removed with the use of a wire brush, mechanical wire brush,
abrasive papers and/or scrapers.

109



Copper Extremely ductile, malleable, good Available in tubes, sheet,
(non-ferrous) conductor. Tarnishes and oxidises quickly. wire, rod and flat bar. Used
Easily damaged and must be stored for water pipes, electrical
carefully. wiring and roofing.

Aluminium Extremely malleable, ductile, lightweight Some types of window
(non-ferrous) and conductive. Non-toxic and often frames. Excellent for
alloyed. Highly resistant to corrosion. stamping and forming.
Can be dyed (anodising).

Lead Very soft, malleable, heavy and highly Roofing
(non-ferrous) resistant to corrosion. Tarnishes to a dull
grey when exposed to air. Very poor
conductivity. Poisonous, so care must
be taken.

Galvanised Highly resistant to corrosion, as alloyed Girders, frames, roofing,
steel with zinc to protect the iron. Can support beams, piping, etc.
(ferrous) withstand saltwater, moisture, rain, snow, Available in tubes, sheets,
etc. Lightweight, fire-resistant, basically ropes and flat bar.
maintenance free and extremely durable
and resistant to scratches and abrasion.

Cast iron Corroding metal, non-toxic, made by Bridges, buildings, stairs,
(ferrous) melting pig iron and small amounts of handrails, cast iron columns,
scrap steel. Strong, hard, self-lubricating items such as machinery parts
and brittle but also cheap, well wearing
and sustains heat.

Wrought iron Iron alloy with very low carbon content Roof trusses, ornamental
(ferrous) made by melting porous iron with slag ironwork, pipe work,
and other impurities. Gives it properties handrails. Available in bar
not found in any metal. form, sheets, rods and hoops.
Tough, malleable and ductile. Can crack
if bent or heated up and brittle when
cold. Has a rough texture so it can hold
platings and coatings.
Mild sheet/ Iron alloy, corrodes and has high carbon Girders, tubes, screws, nuts
steel content so vulnerable to rust. Malleable, and bolts and garage doors
(ferrous) ductile and tough with high tensile (use composites for wood
strength and bends easily. apppearance).

Table 19.11 Non-ferrous and ferrous metals

110


Corrosion Key terms
Anodising – an
During your inspection of the work surface, you may notice areas electrochemical process that
where the surface has corroded, usually due to rust. This will have converts metal surfaces into a
to be cleaned and removed before work can be carried out. decorative, durable, corrosive-
resistant, anodic oxide finish.
Main corrosion factors Aluminium is the metal
Corrosion is the destructive attack on a metal from its often used, but titanium and
environment. The main corrosion factors are all found in the magnesium can also be used.
These metals are immersed in
atmosphere. They are:
an acid electrolyte bath with an
l oxygen electric current running through
l hydrogen
the medium

Unit 2019
l moisture Pitting – formation of small
pits in a metal surface as a
l pollutants.
result of corrosion. Crevice
The most common form of corrosion is oxidation. The atoms in corrosion occurs where
metal combine with the atoms in oxygen to form oxides. Iron rust nuts, bolts and gaskets have
is the most recognisable form of corrosion and is caused by iron been used
oxide appearing on iron or steel components. Metals with high
iron content will corrode more than metals with low iron content.

Metals which show signs of corrosion, such as pitting, must be
repaired and the metal protected with a coating.
Corrosion can protect some metals, such as copper.
Copper is used as both a pure metal and an alloying material.
Because it is very resistant to corrosion, it does not need to
be treated with protective coatings as most other metals do. It
develops a protective oxide coating. This thickens the copper and
turns it green. Did you know?
On iron and steel a thin, flaky
Galvanic corrosion and cathodic protection black iron oxide called millscale
Electrolysis is an electrochemical process, where a metal comes can form (see page 113). This
into contact with an electrolyte (a conductor, usually water) is an example of cathodic
protection to the steel.
and parts of the atoms of the metal (electrons) flow from the
metal into the electrolyte, causing it to corrode. This is called
the galvanic action. If two metals are in the same environment,
the metal that has less resistance to electrolysis (the anode) Find out
will corrode before the other metal (the cathode). Some metal Use the Internet and other
coatings have metal in them lower than the actual substrate resources to try and find some
being painted. This provides protection to the substrate, as the examples where electrolytic
corrosion is used.
anode in the coating causes it to corrode first. This is known as
cathodic protection.

111


To use an example from Table 19.12, if iron sheet has a zinc
coating, this will protect the iron as the zinc will rust first. The
zinc is less resistant than the iron and therefore is the anode.
Copper
Tin
Lead
Nickel
Iron
Zinc
Aluminium

Magnesium

Table 19.12 Metals in order of their resistance to corrosion

Appropriate primers and their function
Primer Description

Mordant solution Also known as etch primer or T-wash. Very toxic and used for
Key term non-ferrous metals. Must have an overcoat after application
to avoid deterioration. Available in two-pack coatings, which
Blast cleaning – an have better stability and adhesion but a limited shelf life once
alternative way to remove mixed. Touch dry within 1–4 hours and can be recoated in 10
corrosion and coatings from to 14 hours.
steel or metal work. It involves
Metal primer Specially formulated to prevent rust and provide adhesion,
grit particles being shot through
(acrylic-based) low VOCs, non-toxic, odourless. Expensive to purchase and
a hose under high air pressure,
can only be used on properly prepared surfaces. Can be
which removes everything it recoated in 4 to 6 hours.
hits on surfaces. This method
is used in heavy industry and in Zinc phosphate Solvent-based primer with a rust-inhibitive pigment,
touch-dry within to 3 hours. Suitable for non-ferrous metals,
fabrication shops.
iron, steel and blast-cleaned surfaces.

Table 19.13 Primers and their functions

Preparation processes for metal
Solvent wiping
Solvents (for example, white spirit and turps) remove grease and
oil from metal prior to decoration. Solvents are used to avoid rust
on the surface. Solvents are very toxic so make sure you are in a
well-ventilated area and that you are wearing the correct PPE.

112


Removing rust by hand
Cleaning off rust by hand is normally done when repainting rusty
steelwork, as it is usually the cheapest method. The problem with
hand cleaning is that the use of scrapers, chipping hammers,
wire brushes and abrasives will not remove all traces of rust. In
addition, the overuse of a wire brush can serve only to polish the
rust on the surface, affecting the ability of the primer to adhere to
the surface. Figure 19.12 A wire brush can
be used to remove loose rust
Follow this procedure when cleaning by hand.
l Remove any traces of oil or grease to avoid spreading it around
the surface.

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l Scrape off all loose rust, millscale and previous coatings.
Key terms
l Use a chipping hammer around rusted nails, bolts and rivets.
Millscale – a thin flakey black
l Use a wire brush to remove loose rust, but avoid burnishing.
iron oxide formed on iron
l Finish off by abrading with a rough aluminium oxide abrasive – and steel. It is also a cathodic
P40–P60 (see page 104–106). protection to the steel

Removing rust with power tools Burnishing – polishing

Power tool cleaning is generally quicker and more effective than

hand cleaning and will extend the life of the paint system. Loose
rust, millscale and the existing surface coating can be removed
using power wire brushes, grinders and needle guns, although
some millscale will not be removed even with power tools.
Again, care should be taken not to over-polish the surface or the
adhesion of the primer will be negatively affected.
Follow this procedure when cleaning with a power tool.
l Remove any traces of oil or grease from the surface. Figure 19.13 Power tools
l Scrape off all loose rust, millscale and previous coatings. such as needle guns and angle
l Use a needle gun to remove rust around corroded nuts, bolts
grinders can be used to remove
rust from surfaces
and rivets, etc.
l Select the most effective method of removing rust to suit the

nature and condition of the surface (for example, rotary wire
brush, disc sander or angle grinder).
Steelwork
l Dry abrade using emery paper or a scraper and wire brush, and
dust off.
l Apply good general purpose metal primer or zinc phosphate

to areas where rust has been removed (apply a full coat for
previously painted steelwork).
l Apply undercoat.

l Lightly abrade, dust off and apply a gloss coating.

113


Key term Other metal surfaces
Bitumen – a heavy, semi-solid,
Ferrous metals (iron and steel)
brown-black substance created
as a result of the oil refining l Remove all corrosion and millscale via mechanical means.
process (also known as asphalt l Degrease with white spirit if necessary.
or tar)
l Allow the surface to dry thoroughly and apply primer with

a brush.
l Bitumen-coated surfaces will require sealing with shellac

knotting solution or aluminium primer.
l Four coats of paint will be required to achieve adequate

film thickness (as recommended by the British Iron and
Steel Association).

Non-ferrous metals (aluminium, copper, zinc,
brass etc.)
l Degrease surface with white spirit.
l Galvanised and zinc-sprayed surfaces should be treated with

mordant solution.
l Etch the surface with wet and dry abrasive paper and white

spirit to provide a key.
l Apply one coat of metal primer or universal primer.

Working life
Jamila has given a client an estimate for repairs to discoloured, flaky and rusty metal
railings. She states that the railings can either be removed and abrasive blasted, then
treated and a full paint system applied or they can be prepared by hand and spot
primed before a paint system is applied to them. There is a cost difference between
the two because of the timescales involved.

Which system should be chosen? What has caused the deterioration of the metal
railings? How would the metal be treated after being blast cleaned? What suitable
paint system could be used? Which estimate should the client go ahead with?

K3. Preparing trowelled finishes
and plasterboard ready to receive
finishing systems
Did you know? Plasterboard is a durable and high-quality lining for walls and
Plasterboard can also offer extra ceilings, lift shafts and stairwells, corridors and auditoriums.
moisture protection as it has Trowelled finishes are surfaces constructed from bricks and
moisture controlling and
blocks. Most walls use bricks and blocks as a central part of
water-resistant properties.
their construction.

114


Types of surface and applications Key term
Gypsum – a white rock
Plasterboard produced as the by-product of
Most plasterboard is made from gypsum, processed into a board industrial processes
and given a paper covering. Standard plasterboard is suitable for
most applications and is compatible with direct decoration or
plaster finishes. It has a grey facing and an ivory coloured back.
Plasterboard is used in a range of residential and commercial
buildings.
Dry lining is where dividing walls in a building are made from
plasterboards (square and feather-edged) which are attached to
timber structures (stud walls) and taped up. The joints of the two

Unit 2019
boards are sealed with joint tape with the joint then plastered over
for a smooth finish.

Blockwork
Concrete blocks are heavy but produce strong finished work.
They are used where a lot of weight will be put on top of, or Figure 19.14 Plasterboard
against, the wall. They are also used to form footings below
ground on walls that support steel. Lightweight blocks are lighter

versions of concrete blocks, produced in response to health and
safety restrictions on lifting and handling units heavier than 20 kg.
Block Appearance Uses

Solid block Concrete block used for making walls
above ground for commercial, industrial
and leisure buildings. Also used for beam
and pot floors.

Hollow block Concrete block used where reinforcement
is needed. Same finish as solid blocks but
with hollow sections running through
them. Filled with vertical reinforcement rods
and concrete, making them very strong.

Aircrete block Lightweight block made of a microcellular
composition (aircrete). Lightweight but very
durable. Used for foundations, beam and
block floors and internal and external
cavity walls.

Table 19.14 Types of blocks and their uses

115


Brickwork
Bricks are smaller than blocks and so more are required per
er square metre. A brick is 215 mm long, or half the length of
65 mm Hea
der Stretch
a block, and 65 mm high. The length of a brick is called the
mm
102
.5 m 215 stretcher and the end of a brick is called the header.
m

Like blocks, bricks are held in place by mortar, a mixture of sand,
Figure 19.15 Sizes of brick, cement and water used for bedding and jointing. Bricks, like
Client: Harcourt
Job No: J6598

header and stretcher blocks, need to be laid level and straight with equally sized joints
Fig No: AW078

to achieve a sound wall with a good appearance.

Physical properties of plaster and

trowel surfaces
Plaster, brickwork and blockwork share a number of physical
properties with metal and timber. Plaster has qualities of
capillarity, tacility, adhesion and porosity. Brickwork has qualities
of capillarity and porosity.
Property Description
Key terms
Acidity Found in some surfaces and can cause defects if the surface has
Alkaline – having a pH
not been prepared and primed correctly
greater than 7 (an acid has a pH
of less than 7) Inertness Refers to plaster being able to bond to surfaces without reacting
or causing defects
Saponification – a chemical
Soluble salt content Salt found in brickwork. It must be removed correctly or will
reaction that makes soap and
keep returning and lead to efflorescence (see page 117)
so foams up as a result

Permeable – allowing things Table 19.15 Some physical properties of plaster, bricks and blocks
to pass through
Alkalinity
The chemical nature of surfaces such as concrete, cement
Acrylic
coating
rendering, asbestos sheeting and some plasters is alkaline.
This can cause problems if a solvent-based paint is applied
Alkali
surface
as the alkalinity in the surface can attack the paint, causing
Alkalinity
permeating saponification.
coating
To prevent this, you should apply an alkali-resistant primer.
This forms a barrier between the surface and the paint. The
permeable nature of acrylic surface coatings means they need an
alkali-resistant primer to prevent alkalinity coming through if the
Figure 19.16 Alkalinity surface becomes damp.
permeating through an acrylic
coating

Harcourt Education
J6637 Painting and
Decorating
9pt Zurich BT
AW034

116


Defects associated with plaster and
trowel work
Like timber, plaster and trowel work can suffer from cracks, nail
heads and open joints. Moisture must also be removed from
plaster and trowel surfaces to avoid mould growth and many of
the defects covered below.

Settlement cracks, shrinkage and dry out Figure 19.17 Settlement cracks

Settlement cracks appear in plaster and cement work in floors and
ceilings. They are caused by shifts in the elevation of a structure
caused by shifts in the soil. The soil may not have been compacted

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properly before construction and, as it decays, collapses – leaving
a void beneath the building.
Shrinkage is caused by material drying out after completion and
cracking as it shrinks. In cement, if too much water is added to
the mix then it can shrink once this water has evaporated.
Plaster that has been properly adhered will avoid shrinkage.

Efflorescence

Efflorescence can be seen as the white patches on cement-based
surfaces and it can occur on brickwork and plaster. Because
cement is porous, moisture can penetrate it, dissolving some
of the lime and creating calcium hydroxide. This then rises to
the surface when the cement dries out, leaving white patches of
calcium carbonate.
Efflorescence will have to be removed before decoration. Scrub
the surface with a stiff fibre brush or a wire brush. Never remove
efflorescence by washing the surface as the calcium carbonate will
dissolve in the water and sink back into the cement.

Defective rendering and raking out
Rendering is a coating of plaster applied to stonework. Cracks can Figure 19.18 Efflorescence
form in this.
To repair small cracks:
l scrape away any loose coatings and particles of masonry
l apply filling agent – exterior grade filler (polyfiller type) could

be used but this would probably re-crack after a short period
of time, whereas exterior acrylic caulking will provide more
permanent flexible repair.

117

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