The final hard-dry time is chiefly dependent upon the binder, although it can be influenced by the solvent. These coatings readily wet surfaces that resist wetting by water, and thus reduce the possibility of such defects as pin-holes. Oils and Oil Paints. The simplest solvent-based coatings are drying oils and their pigmented versions, paints. Drying oils, obtained chiefly from vegetable sources, are those oils that absorb oxygen and solidify within a usable time when exposed to the air in thin films.
Linseed oil is the most common.
Drying oils do not usually contain solvents if used without pigment, but they are solvent-soluble. Because the polymer is formed from small liquid molecules after application, paints take a long time to dry and to cure completely. Because of the increased viscosity some solvent can then be added to help decrease the setting time of the film through evaporation. Oils dry by reacting with the oxygen in the air to form a chemically linked network of molecules. Since oxygen is always available, the reaction can continue past the optimum point so that the film eventually becomes quite brittle.