In addition to burning off with the blow lamp, old-paint films can be stripped by means of paint removers. These are of two kinds – alkaline paint removers and paint solvents.
Alkaline Paint Removers are based on such materials as caustic soda, potash, lime, etc., used with a coarse flour or starch paste to make them easier of application. The alkali combines with the oil in the paint, saponifying it (1.e. turning it into a soap), and softening it so that it can easily be stripped with a knife. In this, they are usually very effective, but though they are cheap their use is not generally to be recommended: when employed to strip paint from woodwork, they tend to leave in the pores of the wood a residue which even repeated rinsing with clean water fails entirely to remove, and which is very liable to attack the new coat of paint. In addition, they tend to raise the grain of the wood, involving additional sanding if a good finish is required, and are often unpleasant to handle and may injure the skin. As they usually damage bristle, it is necessary to apply them with a fibre brush.
These are more expensive but much safer to use. They are prepared from such solvents as acetone, tetrachlorethane, etc., and usually incorporate a proportion of wax to prevent them running too freely on vertical or tilted surfaces, and also to retard the evaporation of the solvent which is highly volatile. Those containing wax are apt to leave traces of it on a surface which has been stripped and hence, before repainting, it is necessary to wash the surface with petrol, turpentine, or other suitable wax solvent, but otherwise they leave behind no substance which can injure the new paint film.