Decorating: the right tools for the job
It is important to choose your painting weather as carefully as you choose your paints and brushes.
There is one limiting factor when you paint exterior surfaces-the weather. The ideal day to paint outdoors is a dry warm slightly overcast day with a drying breeze blowing. This weather is most likely in late spring or early autumn.
Very hot or wet weather does not produce conditions suitable for painting. It is first necessary to look at the properties of paints and the effect of temperature and moisture on paint.
An ordinary gloss paint used on exterior wood or metal work consists of a pigment, a binder and a solvent. The pigment gives the colour and covering capacity, or opacity, of the paint. The binder, a sticky fluid, ‘binds’ the pigment particles together to provide an adherent, a weather-proof layer, and the solvent helps to lubricate the binder and make it run easily under the brush to provide an even ‘cover’ coat.
As the paint dries the solvent evaporates leaving a sticky layer which flows into a smooth film. Paint dries by taking in oxygen from the air; a chemical reaction is set up which causes the paint to set into a smooth, hard film.
If the weather is very hot, the solvent dries rapidly, preventing the paint film from flowing and levelling out properly. This means that some parts of the film, particularly at the end of brush marks, will be thinner than others. The finished surface will be patchy and the paint life only as long as the thinnest parts.
Strong sunlight can cause paint to blister. This happens when sunlight on still-wet paint causes any air or water trapped under the surface to expand and push the surface up in blisters.
Blisters or stickiness may also become apparent when timbers with a high resin content are exposed to strong sunlight. The trouble may occur immediately or appear sometime later. This is daused by the natural resin in the wood and particularly in the knots. Although it consists of the same type of resin as in the binder, the natural resin does not combine with oxygen and dry, as does resin in paint. The result is a wet patch which, as time passes, turns brown and disfigures the paint surface as more resin oozes from the knots. The remedy is to treat knots and any suspect areas with two thin coats of a knotting compound. Allow this to dry before priming the surface.
Low temperatures also affect painted surfaces. If the weather is cold the paint will become thick and difficult to brush out. It is more difficult in cold weather to apply a smooth, even film of paint. The solvent will dry out but the danger is that although the surface will appear to be dry the lower layers will still be wet and soft. The result is that the hard top surface may wrinkle or even crack.
Fog and frost leave a film of moisture on the surface. Attempts to wipe this off may not be successful, especially if the surface timber, possibly only undercoated, has absorbed some of the moisture.
Water under the paint surface can cause a great deal of trouble. A paint film is not impervious to water and in a shaded position the water will slowly evaporate through the paint surface, without causing blistering.
If the surface to be painted is even slightly wet, the top coat may not adhere to the undercoat. While it may appear to dry normally, with fluctuations in temperatures the top and undercoats will expand and contract, causing the paint film to crack and peel off. To prevent this, make sure the surface to be painted is absolutely dry.
No one chooses to paint in the rain. If rain should fall on newly painted work it will depend on how dry the paint film is as to any damage. A quick shower on ‘touch-dry’ paint should do no harm, but rain on a very liquid paint film may thin certain areas and produce a very patchy appearance when dry.
The same rules apply when using any of the newer, polyurethane-based paints which do not require special priming or undercoating when used on timber. These paints are applied ‘coat on coat’.
On metal, a primer must always be used on the base, whatever paint is used.