Problem substrates

Difficult hardwoods

1. Hardwoods such as oak and teak should be treated with care and advice sought. Oak pores often need to be filled while teak frequently has an oily deposit on the surface which should be removed with solvent.
2. Several hardwoods including teak contain extractives which may retard drying. On a new or unknown species a test area should be coated.
3. Unpainted oak sills should be scraped back to a good clean oak surface. Remove imperfections then prime fill and paint as normal.

Plywood

1. Retaining many of the characteristics of wood, plywood should be treated with preservative wherever non-durable wood species have been used in its manufacture.
2. Paint performance can be unsatisfactory due to fine checking appearing on the surface but grades of plywood are available with an impregnated paper overlay which largely overcomes this problem. It should be noted that coated-plywood performance is very dependant on good edge sealing.
3. Woodstains are less affected by checking characteristics but may offer less protection. Although plywoods usually have one or both faces sanded as standard it is possible to obtain rough surface plywood which is more suited to stains.
4. All exterior grade plywood adhesives contain water-soluble salts which are brought to the surface by moisture - this is more likely to occur with woodstains than with paint. In areas sheltered from the rain white deposits build up - they can be removed by washing but will reappear until the source of the salts is exhausted.

Building Boards

The term building board is a very general term which covers a wide variety of materials. Building boards can be made of wood, compressed sawdust or vegetable fibre gypsum plaster etc.

Many building boards are supplied pre-primed or factory finished, some are treated with fire resistant chemicals in which case it is important to seek the board manufacturer's advice before painting.

Most building boards will be hidden from view in that they will be plastered over or used as flooring or as an insulation material in which case the board itself will not be painted.

Some of the building boards listed here may no longer be manufactured but it is still worth mentioning them for the odd occasion that they may be encountered particularly in older buildings.

Where building boards have previously been painted, the actual substrate becomes less important in that you are specifying painting recommendations for an existing paint coating (e.g. Gloss or Emulsion) not the board itself and it is the compatibility of the paint coatings which becomes the important issue. Surfaces that have previously been painted with conventional decorative solvent-based paints such as gloss or eggshell should not be repainted with normal emulsion paints particularly in areas subject to frequent or heavy condensation. The exception to this rule are the quick drying water-based gloss and eggshell paints as they are formulated using a unique adhesion promoted latex which enables them to adhere well to this type of surface.

Emulsion paint may be overcoated with a conventional decorative solvent-based system such as gloss or eggshell if it is considered necessary for improved durability. When specifying solvent-based paints for use on large surface areas inside or in small confined spaces inside the wearing of air supplied breathing equipment by the decorator will normally be necessary in order to comply with a COSHH Assessment as required under the Control Of Substances Hazardous To Health Regulations 1994.

Chipboard also known as 'particle board' is made by compressing wood waste with synthetic resin glues. As a building board its main use is flooring. Chipboard must not be used externally or in areas subject to heavy or prolonged condensation. Some types of board may be impregnated with a fire retardant treatment and should not be painted without first seeking the advice of the manufacturer. When used as a flooring material chipboard is not normally painted.

Composite board is a general term that covers many types of blockboard, hardboard, laminboard, plywood and even plasterboard, it comprises of an outer skin glued to an insulating material such as cork mineral fibre board or polystyrene. Some types have an aluminium foil backing as a vapour check.

Compressed cork is granulated cork which has been compressed and then baked to form slabs for flooring or insulation purposes. Cork tiles are made in the same way. If cork is to be decorated the normal method is to varnish it to enhance the grain.

Fibre board falls into two groups. Hardboard which is a high density smooth faced wood-fibre material and medium board which is less dense and slightly more absorbent than hardboard. The normal use for these boards is internal linings and door facings. Both types are available with a fire retardant treatment. Any board that has a fire retardant treatment must not be painted without first obtaining specific advice from the manufacturer. See also BS1 142.

Fibre insulation boards are a soft low density type board made of wood fibres. They are normally used for thermal or sound insulation and are very porous and so not suitable for external use or areas subject to condensation. Some types of insulation board have a paper face which makes decoration easier.

Hardboard is available in two different types the normal high density smooth faced variety and an impregnated grade. The latter can cause problems in terms of poor adhesion when painted so the manufacturers advice should be sought before painting.

Redecoration of building boards generally

Before repainting, the existing coating must be identified and its condition / performance assessed. If the surface is in good/reasonable condition then a similar paint system can be specified. General preparation should include scraping back to a firm edge any areas of loose/poorly adhering coating if necessary washing down to remove dirt dust and grease rubbing down with an abrasive paper to provide a mechanical key and dusting off. Bare areas should be primed with an appropriate Dulux Primer. In the case of soft fragile surfaces such as fibre board considerable care is needed to avoid damaging the substrates. Where the existing coating is in poor condition or is not suitable for the environment the surface should be completely stripped and a more suitable paint system applied.

When applying solvent-based paints to large surface areas inside or using in small confined spaces inside the wearing of air supplied breathing equipment will normally be necessary in order to comply with a COSHH Assessment as required under the Control Of Substances Hazardous To Health Regulations 2002. Before specifying a solvent-based system a Dulux water-based equivalent should be considered.