Thorough cleaning of the surface is required to remove most of the contaminants which can impair the absorption, adhesion and subsequent performance of coating systems. A solution of household detergent in warm water and a stiff, non metallic bristle brush is ideal for removing oil, dust, dirt or grease. Thoroughly rinse off any residues and allow the surface to dry fully.
Mould/algae - Fungicidal wash
Treat growths with a fungicidal wash or bleach solution and allow 15-20 minutes to work. Loosen dead spores using a stiff, non-metallic bristle brush, rinse with clean water to remove all residues and allow the surface to dry fully. Heavier deposits may need multiple applications.
Removal of old coatings
Chemical strippers are the most practical way to remove unsound coatings. Do not remove sound coatings, particularly opaque finishes for the sake of it but do test them to ensure they are truly sound. Woodstains must be removed if a lighter shade of woodstain is required. Base stains on new joinery provide approximately 3 months protection and their condition should be assessed before top coats are applied.
Bare timber exposed to sunlight for more than 3 weeks will require the removal of the surface back to bright timber. Mechanical sanding is the only really effective method. Abrasive papers should be used - DO NOT use wire brushes or steel wool.
Cut out decay
Rotten/decayed timber is usually soft or crumbly, and must be completely removed back to sound wood. To ensure that all infested timber is removed, cut out up to an inch (2.5 cm) into sound timber. Newly exposed timber should be treated with a suitable preservative, and any voids should then be filled using a suitable woodfiller.
To prevent decay occurring again where damaged timber has been removed from a rotten area, a preservative such as Cuprinol Trade Wood Preserver Clear (BP) should be applied to saturation. Timber used to splice in for repair, should be either of a durable species or pre-treated with preservative.
Surface contaminations of natural timber such as tannins, gums and oils from hardwood and resins from softwood should be removed with a cloth dampened in a suitable solvent (NOT white spirit or turps).
Play a hot air gun over live knots to draw out as much resin as possible, scrape off the resin and degrease as above. Remove dead knots and fill with a suitable woodfiller.
To ensure even, adequate coating to vulnerable edges, all cills should be rounded to approx. 3mm radius ("pencil round") and all other sections to at least 1-2 mm radius.
To avoid the problems of silicone contamination - cissing/cratering/fish-eyeing - a specialist silicone remover or "digester" must be used. Multiple applications may be necessary.
Mortar/Iron Water staining
This form of staining/discolouration is best treated with a wood bleach (active ingredient oxalic acid). Remove all surface coating prior to application. Where iron staining has occurred, remove the cause of the discolouration. For acidic timbers such as oak, Western red cedar and Idigbo, non-ferrous fixings (e.g. stainless steel) should be used.
Non-ferrous fixings are recommended. Where the use of steel/iron fixings is unavoidable, they should be driven home and the resultant holes filled with a suitable filler.
Timber to be coated should have a moisture content close to what it will be in service. Below 14% for interior timber and between 15-18% for exterior timber is recommended.
Voids or areas of damage in timber should be filled prior to coating. All purpose fillers are not flexible enough for use with wood. Only Woodfillers should be used.
Occurs in plywood when atmospheric moisture causes the salts in the glueline to migrate to the surface in the form of white crystalline powder. The strength and integrity of the plywood is unaffected. Normally rainwater washes the salts away but on sheltered surfaces, such as soffits, they remain as a bloom on the surface. Where efflorescence is present, remove fluffy efflorescence deposits by rubbing with dry Hessian sacking at frequent intervals. Check salts do not return within 48 hours, before proceeding. Remove heavy deposits by careful* manual abrasions taking care not to damage the surface finish of the substrate. The surface can then be re-treated. The condition may recur and repeated washing may be required. Eventually the efflorescence will cease.
- When rubbing down and/or dusting off wear a suitable face mask to avoid the inhalation of dust.
Timber which has been exposed to direct sunlight turns grey and becomes denatured (the surface fibres break down and become loose and friable. Any coating applied to this surface is likely to peel/flake within a relatively short period of time. Once it has been exposed to the weather for more than 3 weeks, a thorough sanding is required, to restore a clean, bright timber surface, before any coatings are applied. Wire brushes and steel wool should not be used.
Blue Stain (sometimes called Sap stain) is a fungus which disfigures the timber with a blue-black stain/discolouration, but does not cause actual decay or breakdown of the timber structure. It cannot be removed from the timber, and if its growth is not halted, more serious deterioration is likely. The timber should be treated with a preservative, and the use of a highly pigmented finish (either a dark coloured woodstain, or a solid, opaque finish) will disguise/hide the discolouration.
Hardwoods such as Teak, Afrormosia and Iroko contain 2oily2 components which can impair the drying, hardening & adhesion of coating systems (particularly solvent-based). The surface must be cleaned using a lint-free cloth with methylated spirits to remove natural oils (white spirit is not effective). In extreme cases, the use of cellulose thinners should be considered.
Loose and flaking coatings should be completely removed from the timber before re-decoration. Flaking can be caused by the presence of moisture, timber movement, old coatings in need of decoration, or contamination beneath the coating film. Loose coatings should be scraped back localised flaking to a firm edge, and the timber sanded/cleaned, prior to further coating.
Putty has traditionally been used for glazing, but it shrinks and cracks, allowing moisture in, and consequently its long-term performance is poor. Modern glazing sealants, often used in conjunction with timber beading, are more flexible and far more effective. Silicone glazing materials should only be applied upon completion of the finishing coats.