Learn : Painting new wood work : Knotting or killing the knot, Application of priming coat, Stopping, Application of finishing coats.
Painting new wood work
Following points should be attended to before painting new wood work:
(i) Only well seasoned timber should be painted otherwise not only shall the paint be spoilt but also due to dry rot the timber is likely to decay early. Also the paint surface will otherwise crack due to uneven shrinkage
(ii) It is advisable not to paint excessively dry wood.
(iii) Paint should be applied to only dry surface.
(iv) The surface to be painted should be rendered smooth, clean and free from rust or dirt.
(v) All nails should be punched in 1/2 cm below the surface.
(vi) Large and loose knots should be cut out and filled tightly with correctly fitting wooden pieces.
(vii) The surface should be knotted, primed and stopped before painting.
Knotting or killing the knot.
It consists in rendering knots in resinous wood incapable of spoiling the paint because of the oil that comes out of them. Knotting should be done before painting. any one of the following ways may be adopted for knotting :
Ordinary or size knotting.
First a hot coat of red lead ground with glue size in water is applied on the knot when it has dried then a second coat of red lead ground in boiled linseed oil and thinned with turpentine oil is applied.
Lime knotting.
In it the knot is left covered with hot lime for 24 hours after which it is scrapped. Then the knot is coated with ordinary or size knotting as explained above. The knot is then rubbed smooth with pumice stone or sand paper.
Patent knotting.
it consists in the application of two coats of varnish made by dissolving shellac in methylated spirit or naptha. this treatment is applied to cover stains or on tarred surface intended to be painted. In one liter of methylated sprit are dissolved 250 gms of shellac and stirred with 15 gms of red lead to give suitable knotting for ordinary purposes.
Application of priming coat.
Priming coat of paint is then applied on the surface. It forms as opaque and hard film filling the pores of wood. It provides a smooth base for the paint. This coat is applied preferably before the wood work is fixed in position. Paint for priming coat constists of one kilogram of red lead and white lead mixed with one litre of oil. When the “priming coat” has dried then the surface is stopped.
Stopping.
After the priming coat has dried, all nail holes, other holes and cracks etc. are filled up with putty.*
Ordinary Putty or Glaziers Putty.
It is a paste of very thick consistency used to fill up nail holes, cracks and depression in the wood before applying paint. It is also used for fixing glass panes in doors and windows etc. it is made by mixing 1 kg finely powdered whiting, 65 gms raw linseed oil and 30 gms litharge. All these are mixed well and beaten with wooden mallets until thoroughly incorporated After kneading it well it is left for about 12 hours when it is kneaded again to give smooth workable paste. Desired pigment could also be added to it if coloured putty is needed. Holes and cracks etc., should not be filled up with putty before the application of priming coat as otherwise the oil in putty shall be absorbed by wood leaving the putty to dry which would ultimately fall off.
When the putty has become sufficiently hard the whole surface is rubbed with pumice stone or sand paper till it gives a smooth surface.
Application of finishing coats.
Two or more coats of paint having the desired colour are applied to the surface prepared by knotting, priming and stopping. Each coat should be applied after the previous one, on having fully dried, has been slightly rubbed with fine glass paper. Only good brush held at right angles to the painted surface should be used for painting. Addition of a little copal varnish to paint for final coat shall enable the paint to withstand weathering better and shall add to the gloss.
When white paint is to be done then white lead should be used for outside work and zinc white for interior works not exposed to weather.