This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Here the operation simply consists in stirring or otherwise agitating the resins and solvent together. The agitation must be continued till the resin is all dissolved, or it will agglutinate into lumps. Heat is not necessary, but is sometimes used to hasten the solution of the resin. The varnish is allowed to settle, and is then strained through muslin. In many cases the resin, such as mastic, dammar, or common rosin, is simply mixed with turpentine alone, cold or with slight heat. Care must in such cases be taken to exclude all oil.
In using varnish, great care should be taken to have everything quite clean, the cans should be kept corked, the brushes free from oil or dirt, and the work protected from dust or smoke. Varnish should be uniformly applied, in very thin coats, sparingly at the angles. Good varnish should dry so quickly as to be free from stickiness in 1 or 2 days. Its drying will be greatly facilitated by the influence of light; but all draughts of cold air and damp must be avoided. No second or subsequent coat of varnish should be applied till the last is permanently hard, otherwise the drying of the under coats will be stopped. The time required for this depends not only upon the kind of varnish, but also upon the state of the atmosphere. Under ordinary circumstances, spirit varnishes require 2-3 hours after every coat; turpentine varnishes require 6 or 8 hours; and oil varnishes still longer, sometimes as much as 24 hours. Oil varnishes are easier to apply than spirit varnishes, in consequence of their not drying so quickly. Porous surfaces should be sized before the varnish is applied, to prevent it from being wasted by sinking into the pores of the material.
Varnish applied to painted work is likely to crack if the oil in the paint is not good; also, if there is much oil of any kind, the varnish hardens more quickly than the paint, and forms a rigid skin over it, which cracks when the paint contracts. The more oil a varnish contains the less likely it is to crack. All varnishes improve by being kept in a dry place. One pint of varnish will cover about 16 sq. yd. with a single coat.
The following recipes give the proportions of ingredients for varnishes in connection with house painting: -