Surface Preparation before Decoration: Wood surfaces

Surface Preparation before Decoration Wood surfaces

Before you can pick up that brush and start applying your favourite color Opens in a new your walls, You need to do some surface prep. I know preps sound boring, but it’s something we have to do if at all we want satisfactory results. In surface prep, the wall material is crucial and dictates the products and methods of preparing the surface. In this article, our focus is wood. We have two types of wood available for construction, Hardwoods and Softwoods.

Softwood Description and use

Softwood timbers such as pine, Cedar and spruces come from coniferous or evergreen trees and are used for construction because, generally speaking, they grow very quickly. This abundance of raw materials makes the wood cheaper to produce. Although they are called softwoods, they can still be quite hard and are capable of bearing weight.


Pine trees are valued worldwide for their pulp and timber and are used to make furniture, panelling, window frames, floors and roofing.
Pine is mainly used for internal work but can be used for external work if treated.


There are many cedar trees grown around the world.
Cedar wood makes anything from pencils and guitars to log cabins and fences. Some varieties are highly scented and act as insect repellent Spruce.
Spruce is used in general construction and for making crates and musical instruments. Its pulp is widely used to make paper.
In construction, it is used for first and second fix joinery work. Internal second fix items such as skirting boards, dado rails, architraves, doors and window frames are commonly made from spruce because it is relatively cheap and easy to work. It is nearly always painted because of the knotty and resinous nature of the timber.

Hardwood Description and use

Hardwoods are more expensive than softwoods because they grow more slowly and so cannot be produced as fast as softwoods. Hardwood timbers are used for interior and exterior work, especially for doors and windows and where a more decorative finish is required. Hardwoods can be painted, but the coating will mask the aesthetics of the grain. Therefore, hardwood items are more commonly finished with lacquer, varnish, or French polish to protect and enhance their grain and be more pleasing to the eye.


Oak is a fairly hard, heavy and dense timber with a high crushing and bending strength. It is used for decorative effects such as internal doors and panelling but can also be used for high-quality cabinet making.
Oak comes in various colours, from a rich honey colourOpens in a new tab. to a yellowish-brown and can be stained or varnished to make it darker.
It can be used for exposed roof beams, skirting boards and panelling.


Beech is used in construction for doors, cabinets and log cabins and is pinkish-brown in colour. It is not as decorative as some other hardwoods, so it is usually painted, lacquered or oiled. Beechwood also makes good firewood and is used for smoking fish and other foodstuffs.


Mahogany is used to make furniture, boats and musical instruments and has a fine, even grain. It is reddish-brown in colour and sought after for its beauty and durability.

Types of timber sheets

Many different timber sheet products are available that can be cut to size for both interior and exterior construction work. Each type of sheet timber requires specific preparation to ensure that it is not damaged. Things to consider when working with sheet timber include:

  • the composition and surface of the board
  • health risks involved when preparing the surface
  • the types of abrasive to use.
  • The correct paint system to be used during the application is the most appropriate primer or sealer for the job.

Timber sheet product Description and use


Plywood is made from thin layers of wood, with the grain running in different directions, glued together under pressure.
Depending on the end-use requirement, the laminates can be glued together with an internal or external adhesive grade.


A hardboard is a thin board usually flexible with a smooth, polished surface. It is composed of wood pulp and wood fibre or another vegetable fibre together with suitable fillers and bonding agents and is densely compacted under high pressure.
Hardboards are hygroscopic, and therefore it is advisable to paint the back and edges before fixing them in place to prevent any moisture from penetrating the hardboard, as this will contaminate it and make it unsuitable for use.


Blockboard is used to make doors, tables, shelves, panelling and partition walls. It is normally used for interior work because of the type of glues used.
Blockboard is a wood-based sheet panel made up of a core of softwood strips that can be up to 28 mm wide. The strips are placed together edge to edge and sandwiched between veneers of softwood, which are then glued together under high pressure. The inner strips are generally made up of lightweight poplar wood or spruce.
To achieve maximum strength, it is important to ensure that the core (i.e. the centre and, therefore, the strongest part of the trunk) runs lengthways.
These timber sheet panels are produced in three layers, with one veneer sheet covering each side, or five layers, with two veneer sheets per side for better stability. This sheet material is seldom used now as MDF has superseded it.

MDF (medium-density fibreboard)

MDF can be used to make display cabinets, wall panels and storage units, cover pipework, etc., during the first fix tasks. It is normally painted and can be ready primed when delivered to sites. This sheet material is a hardboard made from wood fibres glued under heat and pressure.
There are various reasons why MDF may be used instead of plywood: it is dense, flat and stiff, it has no knots in it, and it is easily cut on a machine. It is made of fine particles and does not have an easily recognisable surface grain, so MDF can be painted to produce a smooth, quality surface. It can also be cut, drilled, machined and filed without damage to the surface.

OSB (oriented strand board)

OSB is a type of engineered wood, similar to blockboard, formed by adding adhesives and compressing layers of wood strands (flakes) in specific orientations. OSB may have a rough and variegated surface with individual strips of around 2.5 cm × 15 cm (1.0 × 5.9 inches), lying unevenly across each other. It comes in a variety of types and thicknesses.
OSB’s properties make it very suitable for load-bearing structures in the construction industry, and it is now more popular than plywood. Its most common uses are as a protective covering within walls, flooring and roof lining.

Timber and timber sheets preparation processes

You will need to use various materials and the correct processes to prepare surfaces without damaging them. Cleaning agents, all surfaces must be clean and grease-free to ensure a good finish. On previously painted surfaces, warm water should be used with either sugar soap, washing soda or detergent (a small squeeze of washing-up liquid is a good option because it is readily available). Stubborn marks may require a dilution of household bleach in cold water. After washing down with the cleaning agent, the surface will need to be decontaminated with clean water before decorating.

Solvent wiping

Solvent wiping is the removal of residues from surfaces before applying coatings. It involves using white spirit, methylated spirit or acetone on a cloth to degrease and clean an area. Solvent wiping is convenient because the surface does not need to be decontaminated after use. Solvents are also used to dissolve or disperse the paint film and make a paint mixture thin and fluid enough for easy application. The solvents/liquids used by decorators are white spirits, turpentine and methylated spirits. These are all volatile spirits, so they must be kept away from naked flames. White spirit is used for thinning oil paint and varnish to the correct consistency and wiping paint splashes from surrounding surfaces. It is also used for washing paint from brushes and equipment when tasks have been completed. Turpentine is used to make paint and can also be used to thin paint and varnish to the correct consistency. Methylated spirit is used to make cellulose materials such as shellac knotting and cellulose paint and can also be used to thin them. Equipment and tools must be cleaned if cellulose materials have been used.


Dry abrading refers to rubbing down surfaces during preparation; this type of preparation is one of the main tasks completed to produce a decent finish. Several types of abrasive papers are used when rubbing down and keying surfaces. When preparing treated and untreated timbers, it is important to use the right type and correct grade of abrasive paper to avoid causing damage.

Types of abrasives used in the preparation

Using an abrasive that is too coarse can damage the substrate and leave scratches that may show through when painted. On the other hand, using an abrasive that is too fine may be ineffective at removing or levelling surface imperfections. Badly prepared substrates will require making good before repainting.


Knotting is the term given to shellac/white/patent knotting solutions. These materials are used as sealers to prevent resin, marks and stains from bleeding through the final paint surface. As the name suggests, knotting solution is most often used to seal knots in timber, but it can also be used on other stains such as felt-tip pen marks and tar splashes. Be very careful when using knotting solution as it is highly flammable. It is best kept in a knotting bottle or a glass container, preventing the knotting solution from evaporating and drying out.

Applying knotting solution

All the surfaces should be clean and dry before applying.
The knotting solution is best used with a brush, but a clean cloth can also be used.
Cover the knot or stain it entirely to seal it fully; the knotting solution should dry quickly.
When it is completely dry, the surface coating system can be applied. If a brush has been used to apply the knotting solution, it should be cleaned with a cleaning solvent (see the manufacturer’s instructions on the bottle)
If a cloth has been used, make sure that it is opened up and left to dry before fully disposing.


Priming is the first stage of applying a coating system to a surface and is the foundation of the entire paint system. A coat of primer provides a durable and protective coating that acts as a bridge between the substrate and the rest of the paint system. With porous materials such as softwoods, some primer will be absorbed into the wood, leaving enough binding medium on the surface for the subsequent layers of paint to adhere to. Although it can be applied with a brush, roller or spray gun, a better finish is achieved by applying the primer with a brush, even if further layers of paint will be applied with a roller or spray.


Stopping is the preparation process of filling small cracks and holes; the stopper has a stiff paste consistency. It should be used for exterior work as it is more stable in damp conditions. The stopper is pressed into holes and cracks with a small filling knife and then levelled off.


Filling refers to applying powdered fillers to cracks, holes and indentations on surfaces. Fillers come in single-pack and two-pack varieties. There are also plastic wood fillers available, which are used to fill holes and indentations on timber surfaces to be varnished or stained. Plastic filler is available in a range of colours to match the surface to which it is applied. The most common fillers are the single-pack powder fillers used in the painting and decorating trade. These are good all-around fillers, but they are prone to shrinkage, so be prepared to fill larger holes and cracks more than once to get them level. The powder is mixed with clean cold water to prepare this type of filler until it becomes a smooth consistency. When mixed, the filler is usually workable for 30–40 minutes and sets hard within a couple of hours. It can be sanded back to a smooth finish when dry and ready for decorating.

Two-pack fillers

These fillers are often referred to as two-part or deep-hole fillers. These types of fillers cure by a chemical reaction. When the two parts of the filler are mixed, the mixture sets within a few minutes and is ready to be rubbed down within about 30 minutes. This type of filler can be used for larger repairs, as it doesn’t shrink or crack, but it is harder to rub down (abrade) than powder filler. It is ideal for repairing rotten window and door frames and is very tough – it can be drilled, screwed into and even planned. It can also be used for interior, and exterior repairs but is more expensive than single-pack filler or stopping.

Plastic wood filler

This is a fast-drying filler/stopper for small nail holes and cracks, which can be used on exterior and interior softwoods and hardwoods. It can be rubbed down/ abraded when dry, and it sticks well to bare wood. It can also be coated with wood stain, paints and varnish if using the natural version. The application process is as follows: 1 Select the colour of the filler to match the colour of the timber or stained timber that is to be filled. 2 Use a filling knife or putty knife and fill slightly proud. 3 Allow to dry and then rub down with the grain until the filler is flush with the surface using a fine abrasive (silicon carbide paper).

Safety considerations when preparing timber and timber sheets

There are many safety considerations to be aware of when working in the construction industry. (These are covered more fully in Chapter 1, Principles of construction). This section looks at safety considerations when preparing timber and timber sheet materials.

Manual handling

Try to avoid manual handling if there is a possibility of injury. Suppose manual handling cannot be avoided when working with timber and sheet materials. In that case, you must reduce the risk of injury by following a risk assessment and safe working practices. Timber and timber sheets come in various shapes, sizes and weights, so you need to be very careful when handling them. If you cannot safely handle materials, you should get help from a colleague or use the correct lifting and moving aids. Do not lift or carry anything that you cannot safely move. When applying coatings to timber and timber sheets, you may need access equipment such as steps, ladders, podiums or towers. You must always follow instructions, risk assessments and safe working practices when working at height.

Control of Substances Hazardous to Health (COSHH) Regulations 2002

The COSHH Regulations 2002Opens in a new tab. control the use of dangerous substances such as preservatives, fuels, solvents, adhesives, cement and oil-based paint. These must be moved, stored and used safely without polluting the environment. The Regulations also cover hazardous substances produced while working, for example, wood dust produced when sanding or drilling. Hazardous substances discovered during the building process, such as lead-based paint or asbestos, are covered by separate regulations. If you do find asbestos, inform the site manager immediately.

Waste and storage of materials

When carrying out tasks during the preparation and decoration of timber and timber sheet materials, you must be aware of procedures for dealing with waste and storage of materials. Any debris from preparation tasks should be disposed of accordingly, following all instructions and regulations. Coatings should be used up where possible and returned to their stock pots to be used for future tasks.

Personal protective equipment (PPE)

All employees and subcontractors must work safely. This means wearing any PPE that their employer provides and looking after it and reporting any damage to it. When working with timber and timber sheet materials, you need to wear the correct PPE to prevent injuries. This means safety boots, safety gloves and a hi-vis vest if working outside.


You must make sure that you have adequate ventilation in place when preparing surfaces. This can be as simple as opening windows and doors but may involve using extractors during tasks, depending on the size and duration of the project.

Volatile organic compounds

Volatile organic compounds (VOCs) are materials that evaporate from everyday products such as cleaning products and paint at room temperature. VOC emissions contribute to air pollution and affect the air we breathe. The measurement of VOCs shows how much pollution a product will emit into the air when in use (the product container will have a VOC content label so that you can compare products). Solvents within coatings are the main source of VOCs in the painting and decorating industry. However, the paint industry is trying to reduce emissions, and low-odour paints are now available. VOC levels will vary for different paints and varnishes.

Prepare timber and timber sheet products.

Checking timber and timber sheets for defects

Unless the surface to be painted is sound, coatings will not adhere sufficiently, making it impossible to cover – or if you manage to cover the surface, the coating will very soon peel and crack. Decayed or denatured timber will need to be prepared and treated to make the surface ready for coating.

Decayed timber

When timber has decayed due to wood rot, it should be cut out completely and replaced with sound timber, with the inserted timber joints made well beyond the edge of where the decay appears to end.
An area that has become infected with mould, mildew, or other fungal growth will need to be treated with a fungicidal wash before decorating. The stains will show through the final finish, and the fungi will continue to grow. You will need to follow the manufacturer’s instructions for applying the wash and wear PPE, including a mask, goggles, gloves and overalls.

Denatured timber

When timber is exposed to the weather for long periods, its cellular structure breaks down, leaving the surface dull and furry. This is known as denatured timber, and paint is unlikely to adhere to the brittle surface. It will need to be treated before you can proceed. To prepare the surface for decoration: 1 Rub down the surface with a fine abrasive to remove the dead fibres. 2 Remove any dust with a dusting brush. 3 Treat the prepared surface with a wood preservative and allow it to dry. 4 Apply a coat of raw linseed oil to the surface and leave for 15–20 minutes. 5 Remove any surface oil with a cloth and leave for a week to fully dry out. 6 Repaint surface using the same method as softwood.

Preparing softwood timber and timber sheets for decoration

Softwoods need to be lightly sanded using fine glass paper diagonally across the grain and then finished off by very lightly sanding along the line of the grain. This creates scuffing of the surface and enables the paint to adhere to the painted surface. However, if applying a clear or coloured stain or varnish, do not sand across the grain, as the sanding marks will show through and look unsightly.

A step-by-step process for preparing new bare softwood before decoration


Remove any loose debris such as plaster, nibs (small bumps) and bits of building material using a stripping knife.


Protruding nails or pins should be punched below the surface using a hammer and nail punch before using the appropriate filler.


When the filler is dry, rub the whole surface using a fine abrasive. Remember to rub with the grain.


Dust off the surface with a dusting brush.


Apply two coats of shellac knotting to any knots on the face of the timber, ensuring that the coats are thin and well brushed out with no edge build-up. You will need to go slightly beyond the area of the knot. Allow for drying between coats. This should take about 15–20 minutes.

Traditional oil-based primers should be used for external work, as they are hard-wearing, while water-based acrylic coatings are preferred on internal timber surfaces, as they dry more quickly and give off less odour. When using acrylic coatings, it is possible to re-coat the surface the same day as soon as it is dry. When using solvent-based coatings, you need to wait until the following day to ensure that the primer has formed a solid foundation before applying the next coating in the system.

Preparing hardwood timber and timber sheets for decoration

The cellular structure of hardwood is a lot finer than that of softwood, so the pores are not as open or absorbent. Some hardwoods have an oily or acidic nature/texture. The finer grain on some hardwoods, such as oak and ash, makes it difficult for the paint to adhere to the surface. Because of this, a primer must have special properties to enable good adhesion. Aluminium primers that contain leaf or flake pigment are ideal, as is a type of primer called calcium plumbate. To prepare hardwood for decoration:
1 Abrade using a fine abrasive, remembering to rub with the grain.
2 Dust off using a dusting brush.
3 If the surface feels greasy to the touch, the surface will need degreasing using white spirit.
4 Dry off the surface or allow it to dry naturally.
5 Use shellac knotting to seal any knots (apply two coats and leave to dry).
6 Apply one coat of primer. It is essential when painting timber that all end grains are properly primed to stop moisture from being absorbed into the timber. Before priming, two coats of shellac knotting should be applied to the end grain. Shellac knotting may also need to be applied to knots in hardwood and resinous areas if the surface is painted.

Preparing timber for varnish, oil or staining

Use the following steps to prepare timber:
1 Use white spirit on a lint-free cloth to remove any resin exudation or grease.
2 Lightly rub down using fine glass paper or garnet paper.
3 Use a dusting brush to remove dust and particles. The surface is now ready to receive a coating of varnish, oil or wood stain.

Applying primers

It is recommended that a brush is used to apply primers, as the primer can be applied with more force and will therefore penetrate the surface. This enables the surface to be protected and prepared for the following coat within the system being used.

Wrapping up

surface prep is important. To get results as close as possible to the desired outcome, all blemishes that may alter decorations must be addressed beforehand.

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