Whole success of the painting operation depends upon satisfactory preparation of the surface to be painted and the great majority of defects which occur are due to a faulty preparation. It is essential that the wood should be well seasoned and the surface to be painted perfectly dry.
The surface of woodwork to be painted or polished should be rubbed down perfectly smooth with medium and fine grade sandpaper, all rubbing to be done with the grain. Worked timber should be primed as soon as possible particular on the cut end grain.
New woodwork should be knotted, primed and stopped before giving coats of paint.
Knotting
This process is done before the application of a priming coat to cover all knots in wood to prevent any exudation of resin, or any marks to show through the paint caused by the absorption of the knots. There are three common methods of knotting :
a) Lime knotting
b) Ordinary size knotting
c) Patent knotting
Knots in deodar or other resinous woods must be painted over with hot lime and scrapped off after 24 hours, the knot primed with red lead and glue laid hot and one coat of knotting varnish applied the surface rubbed smooth with pumice stone or sandpaper.
Ordinary size knotting is applied in two coats. The first is made by grinding red lead in water and mixing it with strong glue size used hot. (Dries in about ten minutes). The second coat consists of red lead ground in oil and thinned with boiled oil and turpentine. Patent knotting consists in oil and two coats of varnish made by dissolving shellac in methylated spirit or naphtha. Knotting may be composed of 150 grams of pure shellac dissolved in ½ litre of methylated spirit and when thoroughly dissolved, 15 grams of red lead is stirred in. This is suitable for general purposes.
Stopping in filling up nail holes, cracks and other inequalities to bring the surface to a level. Stopping can be done with ordinary putty made of 2 parts of whiting (absolutely dead stone lime), 1 part of white lead, mixed together in linseed oil and kneaded, (100 grams linseed oil to ½ kg of whiting will also do), after the priming coat of paint has been applied and not before, as otherwise the wood absorbs the oil in the stopping can be of a mixture of 1/3 of white lead to 2/3 or ordinary putty. For varnishing the wood surface can be stopped with hot weak glue size (one kg of glue making about 10 litres of size). When dry, the surface should be well sand-papered.
“Beaumontage” or stopping-out wax is a useful preparation for concealing all defects in floor and wood work generally. It is made as follows:
Put a cupful of common shellac in an iron pot, add a tea-spoonful of resin, a piece of bees-wax (about 25 mm and a teaspoonful of powdered lemon chrome or other colouring powdered matter. Heat until the whole is melted and mix all well. It should be applied hot. This wax will not take stains so it must be coloured to suit the finished work. It sets quite hard.
Prime Coat
Prime coat is the first coat applied to fill the pores of wood or any minute inequalities on the surface to be painted. It also prepares a smooth base for the subsequent coats of paints and accelerates their drying. A priming coat may be given of red lead or of red and white lead mixed in double boiled linseed oil (3 kg of red lead, or 3 kg of red and white lead mixed with 3 liters of oil). When dry, all cracks or holes are filled up with putty and the whole surface rubbed down with pumice stone or sandpaper, and allowed to harden before applying paint. (Priming coat should have no turpentine)
Wood with excess of resin or oils in them are unsuitable for polished and painted work e.g. the resin of Deodar shows itself up in discoloured patches even through a number of coats of pain.
Second coat of the desired colour is laid on in exactly the same manner as the priming coat and when dry the surface is rubbed down with pumice stone or glass paper. This is followed by third coat. One kg of pumice is required for rubbing down about 200 sq. m of old surface.
Each coat is allowed to dry completely before the next is applied. The final coat should be carefully crossed. Paints should be applied in thin coats, thick coat take longer to dry and generally begin to flake off after sometime.
If lead paint has been used, the dry rubbing of lead painted surfaces must invariably be prohibited. Dry sandpapering of painted surface is the cause of lead poisoning among painters. Water proof sand papers or flint paper and cloth for rubbing are available.
When the work to be painted is subjected to a strong light and is not of very high finish, oil paintings show up every defect ; in such cases it is desirable to have the painting done in turpentine instead of oil the result being a flat instead of shiny surface. The proportions used are 1 kg of white zinc, 1/3 litre of turpentine and 1/6 litre of boiled linseed oil.
When white paint is specified, white lead should be used if the work is outside and likely to be exposed to the weather. White zinc should be used for inside works not exposed to the weather.
Blistering : Sometimes paints blister if the coat is too thick or if there is moisture in the paint of the wood painted.
