THE object of painting is to preserve the more perishable parts of a structure from the effects of the weather, heat, gases, etc.
In the best buildings the woodwork receives at least four coats of paint, sometimes five or six; but in those of an inferior class two or three coats only are used.
Each coat, as the work approaches completion, should incline more in tint to the final colour.
Before proceeding farther it is necessary to allude briefly to the materials of which ordinary white lead paint is composed, though the composition and peculiarities of these materials, and other points connected with the subject, will be gone into more fully in Part III.
The paint in ordinary use for protecting woodwork is composed chiefly of white lead, linseed oil, and litharge (or other "driers"); sometimes a little turpentine, and small quantities of other ingredients are added, as hereafter explained.
The part played by each of the principal ingredients is as follows: -
The white lead gives a body to the paint, and combines with the oil to form a kind of soap.
The spirits of turpentine, or "turps," is used merely to save oil, and to make the paint more liquid, so as to work freely. It evaporates, and plays no part in protecting the wood.
Red lead is generally used with the priming coat; it dries well and sets hard.
The proportions given in the following table must, therefore, be taken only as an approximate guide for inside work when the materials are of good quality: -
Raw Linseed OiL
Litharge or Patent Driers.
Number of Superficial Yards the Paint will cover.
1st Coat or Priming................
2d Coat .....................
3d, and remaining Coats
* Or ½ oz. burnt white vitriol, and ½ oz. litharge.
The last two coats have the final colouring added in proportion to the depth of tint required; from 1 to 2 oz. of colouring matter is added for every 10 yards of surface to be painted, and the quantity of white lead is reduced in proportion.
Woodwork should be thoroughly dry before being painted. The surface should be planed clean and smooth, and free from dust. All nails should be punched in, so that their heads are driven below the surface.
The knots should then be " killed " by painting them over with "size knotting" or "patent knotting" (see Part III.) This is necessary, especially with fir and resinous woods, to prevent the turpentine in the knots from exuding through the paint.
There are several other ways of killing knots. Sometimes they are covered with fresh-slaked hot lime for about 24 hours, which is then scraped off; after which they are painted with size knotting, and if this does not kill them they are coated with red and white lead in linseed oil, and when quite dry rubbed smooth with pumice-stone.
Sometimes, after application of the lime, they are ironed with a hot iron, and then painted smooth.
In superior work the knots are cut out to a slight depth, and the holes filled up with putty made of white lead, japan, and turpentine.
Sometimes the knots are covered with gold or silver leaf.
After "knotting" the "priming" coat is laid on. This generally contains a large proportion of red lead, which makes it set harder, and gives it the pink colour familiar to all in new work.
The object of this coat is to fill the pores of the wood before applying the colouring coats, which would otherwise be sucked up and wasted by the wood.
"Painters will sometimes for cheapness prime with clearcole or glue size instead of oil, which form a skin over the surface, without entering into the pores of the wood; it is liable to peel off, and should never be allowed unless the surface is too greasy or dirty to take oil priming."1
When the priming is dry the second coat is laid on and allowed to harden. If the knots still show, they may be covered with silver leaf, gummed on with size. This, however, is seldom done in practice.
The third coat is then applied, and when it is dry and well rubbed down the finishing coat is added.
In good work each coat should be carefully rubbed down with sand-paper or pumice-stone, and well dusted, before the next coat is laid on.
This coat must be laid on quickly, of a tint somewhat lighter than the ground colour.
It does not protect the material to which it is applied, as an ordinary coat of paint would do, for the turpentine evaporates, leaving only the pigment.
" Sometimes a little size, or raw oil well bleached, is added to the turps, in order to enable the paint to stand washing better, in which case it is called bastard flatting." l
If the paint is to be exposed to the sun boiled oil should be used, and the quantity of turps in the second coat should be reduced to about one-half that mentioned on page 195, and there should be no turps used in the remaining coats, except in winter, when a little is necessary to make the paint work freely.
2 Seddon's Builders' Work.