Provided that the appearance and durability of the finish are not impaired thereby, speed in the drying of a paint is manifestly a very great advantage and is one which, in fact, is regarded almost as an essential under modern conditions. It is this property which has been mainly responsible for the extensive use of nitro-cellulose and synthetic-resin finishes for industrial purposes; these types of materials are dealt with at some length later on in this work, in Volume II, but we may here mention a third variety of quick-drying paint – the ‘ wet-on-wet,’ by means of which it is possible, under certain conditions, to apply as many as five coats in a single day.
Special formulations are used to produce ‘wet-on-wet’ paints: the drying interval between coatings (when sprayed) varies from thirty minutes to one hour and, even though each may be of a different colour, no running or mixing takes place. Each coat reaches a semi-solid condition very rapidly and no sagging, wrinkling, or lifting takes place when a new one is superimposed, as would be the case with paints of the traditional kind.
The explanation lies chiefly in the use of specially pre-treated oil which, in this instance, dries not by oxidation but by a change in its molecular structure known as polymerisation. This, together with the type of solvent employed, causes each subsequent coat to knit into the previous one, so that the whole, from primer to finish, forms one homogeneous film of great strength and toughness. ‘ Wet-on-wet ‘ paints are designed primarily for spray application, but at least one can be brushed on though the drying time which must be allowed before another coat is put on is approximately six hours. These paints, it should be added, have now been on the market for several years and are used by many leading industrial concerns with complete success. Though hitherto they have played only a small part in domestic work, there is no reason why they should not be employed for this purpose.