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.
When the surface to be painted is already covered with old paint, this mould be either removed or rubbed down smooth before applying the new. When the thickness of the old coat is not great, rubbing down, accompanied by a careful scraping of blisters and defective parts, will suffice. When the thickness of the old paint necessiates its removal, it may either be burned off, or softened by a solution of caustic alkali, and afterwards scraped. The burning process is the most effective, and leaves the wood in a fit condition to receive the fresh coat of paint; but it is not applicable in the case of fine mouldings. When caustic potash or soda is used, the paint is left in contact with it for some time, when the linoleic acid of the oxidized linseed-oil becomes saponified, and can easily be scraped or scrubbed off the surface of the wood. Whenever an alkali is employed, it is of the greatest importance that the wood should afterwards be thoroughly washed several times with clean water, in order to remove every trace of; he solvents.
Any soda or potash remaining in the pores of the wood would not only retain moisture and cause blistering, but would also have an injurious action upon the vehicle of the paint subsequently applied, and in many cases upon the pigment itself, the remarks already made as to the necessity of an absolutely dry surface should be borne in mind in this instance. When the surface of the paint is to be protected by a oat of varnish, the latter should not be applied until the whole of the oil contained in the paint has solidified. The wrinkling of varnish upon paint is frequently erroneously ittributed to the bad quality of the varnish, when the real cause is the incomplete oxidation of the paint itself. Following are some recipes for removing and cleaning old painted surfaces: - (1) Dissolve 2 oz. soft soap, 4 oz. potash, in boiling water, add J lb. quicklime. Apply hot, and leave for 12-24 hours. This will enable the old paint to be washed off with hot water.
(2) Cleaning old paint is effected by washing with a solution of pearlash in water. If the surface is greasy it should be treated with fresh quicklime mixed in water, washed off, and reapplied repeatedly.
(3) Extract of Lethirium is a ready-made preparation which removes old paint very quickly. For this purpose the pure extract must be thinly brushed over the surface twice or thrice. To remove a single coat of paint the extract is diluted with 30 times its bulk of water. To clean painted surfaces it is diluted with 200 or 300 parts of water. The extract must be carefully washed off with vinegar and water before laying on another coat of paint.
(4) Wet the place with naphtha, repeating as often as is required; but frequently one application will dissolve the paint. As soon as it is softened, rub the surface clean.
Chloroform, mixed with a small quantity of spirit ammonia, has been employed very successfully to remove the stains of dry paint from wood, silk, and other substances.
(5) Mix 1 oz. pearlash with 3 oz. quick stone lime, by slaking the lime in water and then adding the pearlash, making the mixture about the consistence of paint. Lay the above over the whole of the work required to be cleaned, with an old brush; let it remain 14 or 16 hours, when the paint can be easily scraped oft*.