Treatments for Home Exterior
Timber treatment and maintenance
External woodwork is vulnerable to weather and atmospheric pollution. The natural colour of wood is destroyed by ‘leaching’- bleaching by the sun – and by the effect of pollution. Moisture and dryness also cause timber to expand and contract, loosening the surface fibres and allowing a mould to form.
These cumulative effects can be remedied by using timber preservatives, varnishes or water-repellent preparations.
Before treating the timber, first prepare the surface. Most should be brushed down to remove dirt and grit. Preparations are applied with a brush, rag or under vacuum pressure, a commercial treatment. Timber may also be dipped in preservative. Avoid using a spray, unless recommended by the manufacturer.
Preservatives fall into three basic types-water-borne; coal tar; and organic solvents. Some of these are poisonous, so wear protective goggles when applying and immediately wash any splashes off the skin.
Coal tar oils, such as creosote, are used for fences and sheds. It is brushed into the timber.
Water-borne preservatives, which are colourless, are normally applied commercially. Timber so treated has a high degree of protection and can be stained, painted or varnished once dry.
On a decorative timber surface, organic solvent preservatives, coloured or clear, are suitable. They can be painted over when dry.
Water repellents, which may contain preservatives, help to preserve the natural appearance of timber cladding. Dipped or applied with a brush, they can be clear or stained. To prevent moisture penetration, water repellents containing oil and wax are best but cannot be painted over.
Sometimes, natural oils are used on exterior woodwork. To give maximum penetration, dependent on the porosity of the wood, a mixture of boiled linseed oil and white spirit gives the best results.
However, oils do not give the effective protection of good water-repellent preservatives. The advantage of oils is that the grain and natural colour of the wood show through, but the surface, according to climatic conditions, may need treatment twice a year.
Before applying an oil-based preservative, the area should be brushed with a stiff brush and rubbed down with medium glasspaper.
Exterior cleargives an attractive finish and allows the natural timber to show through. Varnish is composed of alkyd, copal or phenolic resins, combined with drying oils. Four coats are needed on new timber. The first coat should be thinned.
This treatment should be renewed at the first sign of any breakdown in the surface. Remove the varnish with a solvent. Clean off with white spirit, to remove any remaining wax in the solvent and then apply two coats of varnish. On a sound but dirty surface, wash down with soapy water and finally wash again with clean water.
Use a wet-or-dry abrasive paper to rub down. Fill any holes or cracks with hard stopping, which should be coloured to match the surrounding area. Stain any bleached areas and then apply two coats of varnish. Varnish should only be applied on completely dry wood, as moisture may cause it to peel off.
Polyurethane varnish can be used externally. Again, a minimum of four coats should be applied, ideally at four-hourly intervals. Choose a dry, warm day, with a temperature of 15-25°C, and ensure that the surface is completely dry.
Polyurethane varnish tends to be brittle and peel away at the edges, and a coat of alkyd varnish may help to prevent this. If the surface has broken down, the gloss must be completely removed, the surface cleaned and new varnish re-applied.
If the surface colour of the wood has faded, it can be re-coloured with a stain. These are of two types. The first, in a water-repellent or organic solvent preservative, is applied initially and fades as the coat wears. These can be washed off.
The second type darkens the wood and should be used with care. This is used on timber which was originally treated with a clear water-repellent preservative, but has lost its natural colour.
External timber surfaces that might need treatment include cladding, timber window and door frames, external doors, garage doors, sills, thresholds, greenhouses, sheds, gates and fences.
On timber cladding to be painted, use an alkyd-resin paint in the colour of your choice. On new wood first use a suitable primer. Highly resinous woods, such as cedar and some pines, are better sealed with an aluminium primer. Use one undercoat and two top gloss coats for maximum protection.
When repainting, if the surface is sound, it may not be necessary to strip back and reprime. Wash down the surface, allow to dry and then rub down with abrasive paper. Wet-abrasive rubbing is easier and prevents too much dust from flying about.
Any blemishes or blisters in the paint should be rubbed down and re-primed. Knots or resinous patches should be treated with patent knotting. Holes, cracks, or joints should be filled with hard-stopping, where necessary, and rubbed down.
Badly blistered or cracked surfaces must be stripped or burnt off and the area treated as new timber. An outdoor grade of emulsion, with an appropriate primer or primer-sealer, may be used on cladding.
Aluminium cladding may be painted with an undercoat and two top coats of alkyd-resin paint. Before painting, clean the surface with white spirit and touch in any worn areas with zinc-chromate primer. To prepare the surface for painting, brush off dust and any loose material. If any mould or lichen has developed, wash down with a fungicidal solution, allow to dry and then brush off the growth. Apply an alkali-resistant pigmented primer before applying the top coat. Lime wash or cement paints do not need a primer but the surface should be wetted before the first coat is applied.
Some paints are highly resistant to alkali attack. When using a paint that is not highly resistant, use two coats of primer to prevent the alkalis seeping through to attack the top coat.
Primer sealers can be used before painting to counteract uneven suction but will not prevent alkali attack. Sheeting or soffits should be either painted with porous, alkali-resistant paint or with an impermeable paint where the sheet has previously been ‘back painted’.
Down pipes and the insides ofshould be painted with bitumen paint. Allow this to dry thoroughly before painting the outside. Applying bitumen to the insides of pipes is difficult and is usually done during production.
10. November 2011 by admin
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