A distinguished British painter declares: "The less paint that is put on at each opera tion, consistently with a proper covering of the ground, the better will the ultimate result be.° "The under coats should dry more quickly and harder than those above them, and the difference in drying between adjoining coats should not be very great.° From experiment with an ocular micrometer in connection with a microscope, it is found that single coats of dried paint vary in thick ness from 1/500 to 1/1000 of an inch, the differ ence being due either to the manner of applica tion, i.e., whether under light or heavy pressure of the brush, or to the difference in the consist ency of the paints tested. Few realize the thin ness of coatings of paint or the strains to which its dried films are subjected. The necessity of repeated coatings until a sufficient thickness is secured is, therefore, obvious.
Viaducts, tunnels, bridges, etc., require differ ent treatment from structures exposed chiefly to the action of rain-water and sunshine. Lin seed oil, in drying, as already explained, under goes a metamorphosis; and the result of this process is a solid linoxyn. A film of this dried linseed oil, or linoxyn, is not quickly formed without dry air and light, but once formed is much more stable and better able to resist the agencies that destroy paint than a thin layer of undried linseed oil; in other words, wet paint is much more sensitive than dried paint. There fore, in locations that are ill-ventilated, that get no sunshine, that are damp and filled at times with steam and acid gases, one must have material of a different kind for coating steel. Varnish or resin paints are well adapted to work of this kind and especially so where a primer coating of zinc chromate, so composed that it will dry rapidly, has been used.
Much has been said and written of late re garding the apparent failure of paints of wide reputation when applied to steel cars. This re sult may be due to an insufficiency of paint, or that it has been applied in the open under un favorable weather conditions. Good paint ille gitimately thinned with cheap oils or japans is practically rendered of little protective value, and to apply it to steel cars more or less cov ered with rolling-mill scale, dew, frost, snow, slush, ice, grease, etc., is waste of time and material. There is no reason why paint applied to steel cars should not wear as well as paint applied to any other steel structures, provided it is used under fair conditions, applied by competent workmen and enough time for dry ing is allowed to elapse between coatings. The only way to secure better painting is to employ competent and vigilant inspection of the paint er's work.
Factors that Affect Results in Painting.— (1) Location of the structure, e.g., seaboard or inland; (2) kind and condition of the surface; (3) quality of the paint ; (4) workmanship of the painter; (5) number of coats applied; (6) time allowed to elapse between coats; (7) at mospheric conditions when painting is done.
- Paint tests may be of three kinds, namely, chemical, mechanical and physical tests.
Chemical In the selection of solids and liquids for paint-making, it is well to know that they contain no deleterious matter, such as soluble solids or destructive liquids, nor use less substances either to make weight or to make bulk. Chemistry can tell these things, but it cannot tell the quality of the bulk of paint materials used. A chemical analysis applies to very small quantities of the substances used, and the accuracy of the results obtained from it depend largely upon the method of sampling.
When one considers that about all pigments are allotropic, that no two lots of paint liquids are exactly alike, and that any prepared paint changes more or less, in one way or another, with age, the chemist's test is proved to be of value in so far as it relates to the matter subject to his analysis and determinations, and no further.
Chemistry can tell things that may have been used in making a paint; it can foretell some phenomena that it may develop, but it cannot at all surely predict its "vis viva° — that is, what the stuff will do, and what it will do is the only true measure of its value or worth.
Mechanical By this is meant tests made by a skilful painter. No one can deter mine the working qualities of paint as well as the man whose eye and hand and arm are trained through practice. The importance of the proper application of paint receives less at tention than it deserves, e.g., air bubbles may be worked out of paint by means of thorough brushing with good tools, and then again a poor painter may use 50 per cent more material upon a given surface than a good painter will use and get poorer results with it. Engineers and architects, therefore, demand the employment of competent artisans to do their painting, for results are often more dependent upon the intel ligence and goodwill of the painter than upon the quality of the material used.
Physical By this is meant weather tests, or the exposure of the dried paint, on metallic or other surfaces, to the destructive forces of nature, such as sunshine, rain-fall, frost, dew, heat, cold, light, darkness, etc., or to those agencies which are frequently present in atmospheric air, that shorten the life of dried paint, such as acid or alkaline gases and vapors. Comparative tests on limited surfaces — that is, small plates of steel or glass — are often mis leading, because a painter cannot gauge his work with unfamiliar material, and because his brush probably will not be in condition to give the paint a fair show. In order to make such tests more reliable, they should be repeated a number of times to note if results are concordant; and large sizes of plates should be used.
Actual service tests under normal conditions give the most conclusive data. Accelerated tests under abnormally severe conditions have little value unless the results obtained by the method selected will be in harmony with long time service tests. It is easy to test the water proofing quality of a dried coating of paint, but such a test determines nothing more than the ability of the paint to exclude water from the surface underneath it. It is easy to test the effect of artificial heat upon dried paint, but this does not determine the power of the paint to withstand sunshine. No quick test to deter mine the probable life of paint, under such conditions, is known.
Bibliography.— Andes, L. E.,
Revised by RICHARD FERRIS.