Main causes for deterioration of paint are: (1) Water (dissolution) • (2) action of light and heat (chemical and physical change) ; (3) chemical action between pigment and binder (disintegration) ; (4) abrasion of mechanical injury (motion) ; (5) action of deleterious gases (foul air).
A worthy metal coating sometimes fails: (1) Because of improper application (incom petent or careless users); (2) because of in sufficient quantity on a given surface; not re newed at right time (perishes quickly) ; (3) because of an unstable foundation to stick to (dirt, grease, dampness or scale under it) ; (4 because the under-coating is more elastic than that over it (cracks) ; (5) because not protected when drying (wet paint sensitive).
Painting.— The coating of structural steel with paint is for the special purpose of pre venting air and water from getting at it, as a combination of these will aid in allowing the setting up or continuance of electrolytic action the more quickly if carbonic or some other acid be also present. Many worthy paints are ruined because flowed on with a broad, thin, flat brush, not much better than a common whitewash brush, instead of being rubbed out and into the surface well with a good, full, round, bristle brush; and then, again, painters are apt, if the pigment used is heavy, or the parts to be cov ered are difficult of access, to add an excess of thinners to save time. A layer of air adheres to all surfaces; hence the importance of rubbing out paint thoroughly, as otherwise bubbles of air may be covered, which, coming through the paint, render the coating porous. It is known that the durability of any oil paint de pends largely upon the number of particles of pigment upon a given area of surface, and that the more particles of pigment the better the protection to the binder and to the metal under neath; the evil of thinning too much is, there fore, obvious. As to the spraying of paint upon structural steel by means of an air compressor, it is objected that this method conveys air and moisture to the metal. The adherence of paint will be increased if the metal is moder ately heated before it is primed, because if it is warmed by artificial heat, the surface will be dry. Mill marks? even though made with
inferior paint, generally afford excellent pro tection to steel because the paint was applied to the metal when it was clean and warm.
When painting is to be done in the field, or under conditions that make it impracticable to warm and dry the metal artificially it is a good plan to apply paint heated to a temperature of about F., especially when the temperature of the atmosphere is below 55° F. This warm ing of paint may be attained by placing the pails of paint in vessels of hot water. It is import ant that one should always note the atmospheric conditions when the work of painting is being done. A temperature of about 70° F. and an atmosphere that is free from moisture favor the right kind of drying. Humidity of the at mosphere is even more important than its tem perature. Nothing retards drying more than dampness and darkness.
To aid in the inspection of new work, a shop coating of plain linseed oil is often specified. This does, in a measure, protect the iron from rust, but a very uneven film of dried oil prob ably will be secured from this method of treat ment. Oil applied to a vertical surface runs off until the layer reaches a certain thickness. Where the current meets with an obstruction it piles up into a thick and uneven coating. These thick parts of the layer may require months to harden into a substance firm enough to be fit to paint over. The results are most disastrous in cold weather when oil thickens and contracts. As painters express it, oil, when applied to cold steel, crawls, wrinkles and crinkles. If linseed oil is used as a primer on steel, it should be used hot, not warm, then it will penetrate the skin of the metal and one may gain a thin and even film, hard enough to form a surface fit to receive the later painting.
Many places such as the under and inner sides of girders, bolt-heads and nuts, rivets, etc., which should be painted with great care to prevent the incursion of water, are often over looked or neglected. To aid inspectors, a proper check upon the workmanship can be secured by selecting different colors for the several coat ings, so that uncoated or defective places may be readily detected.