New woodwork requires to be knotted, stopped, primed, and in addition painted with two coats of undercoating and one coat of finishing oil paint. If paint were applied over the bare knots of new wood it would be destroyed, or at least discoloured, by the exudation of resin from the knots. This is prevented by coating the knots with knotting varnish, made by dissolving shellac in methylated spirits. The purpose of the priming coat is to fill the pores of the wood and to furnish a foundation. The priming coat is a thin paint made by substitut ing turpentine for part of the oil in ordinary paint, and dries with a flat hard surface.
After the priming coat is thoroughly dry, all nail holes and cracks in the wood should be stopped with white lead putty; and when hard, sandpapered to give a smooth even surface. Two coats of undercoating paint are then applied to give a solid foun dation for the final finishing coat.
All structural iron and steel work must be thoroughly cleaned from scale and rust before painting by the use of hammers, scrapers and wire brushes, and all grease care fully washed off with benzine. Three coats of paint should then be applied. The priming coat should be red lead paint and the subsequent coats may be either white lead, basic sulphate of lead, or red oxide paints.
The method of applying paints by the dipping or spray process is now largely used industrially, as it enables painting work to be finished much more expeditiously than by the ordinary brush process. In the spray process the paint is applied by forcing it under pressure through a spray-pistol ; the dipping process consists in immersing the objects in the
and subsequently hanging them up to dry. For use in these proc esses it is necessary that the paint should be of a thin consistency. Painting on Cement and Plaster Work.—Paint should not be applied on new cement or plaster surfaces as they contain free alkali which is destructive to paint. The best treatment is either to leave the work unpainted for about 12 months, when most of the free alkali will have disappeared, or to apply a coat of distem per. If it is essential that the work be painted immediately, then the free alkali may be neutralized by washing thoroughly with a solution of 3 to 4 lb. of zinc sulphate in one gallon of water.
Paint will not adhere to new galvanized iron, because it has no grip on the smooth greasy metallic surface. To overcome this difficulty new galvan ized surfaces should be treated with a solution containing 4 oz. of copper acetate, or copper chloride in one gallon of water.
Troubles.—Slow-drying. This may be due to several causes.
(i). The use of insufficient driers. The quantity of driers re quired to make a paint dry in a reasonable time depends upon its composition, and also on the weather conditions when the paint is applied. Dark coloured paints, such as black paints and purple oxide paints, require considerably more driers than white lead paints, also more driers are necessary when painting in the winter than in the summer months. Painting work should never be done during damp, cold, foggy weather, as otherwise the paint may take several days to dry.
(2). Faulty preparation of the work. Paints will not dry on greasy or dirty surfaces, hence it is absolutely essential to take care that all the work has been thoroughly washed down and cleaned before paint is applied.
(3). The use of adulterated oils. Paints made with adulterated oils such as mineral oils, paraffin oils and resin oils, will not dry off hard, but always remain soft and "tacky." Blistering. The cause of paint blistering is often due to painting on damp work or timber which has not been properly seasoned.
Blisters may also be caused by the use of too much oil in paint exposed to heat, or the application of one coat upon another be fore the latter is properly dry.
Cracking is caused by the use of too much oil in the undercoats and too little in the top coats. It may also be due to the paint containing insufficient oil, or a resin varnish.
Loss of Gloss. If a paint is applied on a porous surface, or one that has not been properly primed, it will dry with a poor gloss owing to the paint "sinking in." Paint when applied in damp or wet weather often loses its gloss and dries with a dull finish, owing to the action of the moisture on the paint film during the process of drying.
See W. J. Pearce, Painting and Decorating (1927). (J. G. BE.)