Practical House, Wagon and Automobile Painter / including sign painting, and valuable hints and recipes

by W. F. White

The subject, as I understand it, relates to the cracking of varnishes, etc., as experienced in connection with passenger car work, and as such I introduce it for discussion before this association.

There are many theories as to the cause of the cracking of paints and varnishes. Some are well defined, others are not satisfactorily explained.

I do not anticipate being able to add much to what is already known, but will advance a few thoughts, which may call forth the views of others on the subject.

The old adage, “It takes two to make a quarrel,” is as true when applied to paints and varnishes as it is to individuals. A single coat of either seldom, if ever, produces cracks. These make their appearance only after two or more coats have been applied; consequently, it is necessary to have a body of color or varnish, consisting of two or more coats, before any trouble of this kind makes itself manifest.

This being the case, it follows that the cause of the difficulty must be sought for in the coatings themselves, either in the quality of the material employed or in the mode of applying them.

Poor and cheap oils and japans—especially the latter—are a fruitful source of cracking in paint; but by far the most prolific one, in my opinion, is the hurried application of the succeeding coats before the preceding ones are dry enough to receive them. If sufficient time is not given, cracks will inevitably follow such a mode of procedure.

I am of the opinion, also, that very little blame can be attached to the wood used in the construction of cars, as most of it is comparatively well seasoned, and its expansive and contractive force is not sufficient to cause serious trouble. If green wood was used there might be room for this excuse, especially where the cracks run in the direction of the grain, and are large and deep.

Before pursuing this subject further, it may be well to examine a little into the theory of the drying of paint. It is purely a chemical process, not a mechanical one, as some suppose. Paint dries by the evaporation of its volatile parts and its absorption of oxygen; it is heavier when dried than when in the liquid form, having attached to itself a sufficient amount of oxygen to very perceptibly increase the weight some 6 per cent.

The best grades of linseed oil are said to contain from 70 to 80 per cent. of substance called linoleine, a resinous and slow-drying oil and acid which imparts to the oil its elasticity.