A pigment that will not hold the oil pro duces a fugitive paint. Any pigment that will lake and retain oil or binding material, and that Is changed by the agencies that destroy dried oil, or the binder, will make a durable paint for structural steel provided it is fairly treated and properly applied to a surface in fit condition to receive paint.
Andes in his work on 'Anti-Corrosive Paints,' in discussing the number of coats of oil paint that must be applied to iron in order to secure efficient protection against rusting, says that in establishing a standard, preference must undoubtedly be accorded to paints requir ing a large proportion of oil to make them work properly. He says: "It may be laid down as a normal standard that, persupposing the use of good paint, .one bottoming coat and three subsequent coatings, laid on at suitable inter vals, will be required to produce a layer of sufficient thickness on iron to keep moisture away from the metal and to protect the latter from rust for a certain time' The first or prime coat upon a surface of steel, in a fit con dition to receive paint, is of the greatest im portance, especially as to its drying, hardening. adhesiveness and impermeability to water.
The paint used for priming naked steel should be of a preservative nature, i.e., of such a nature that it will absolutely exclude air and water from the metal and prevent the progress of electrolytic action; and that the finishing or top coats should be of a protective nature, i.e., of such nature that they will protect the primer or bottoming paint from the action of rainfall, sunshine or any special external condition. As the first or priming coat must form a receptive foundation that will be able to hold all subse quent or protective coatings both on and up, it should be largely formed of basic materials, so composed as to secure on the surface of the metal a firm and unyielding coating of an elas tic, durable, cohesive and adhesive cement; it is probably necessary that this cement be made from a pigment that secures chemical action between itself and the liquid with which it is mixed, for it is only through the use of such a pigment that a skin or covering can be secured upon unheated metal that will be impervious to air and water.
A series of exhaustive tests carried out by the American Society for Testing Materials established the fact that among pigments the best for preserving steel from corrosion was zinc chromate, known commonly as zinc yel low. Following this, and in the order of rela
tive efficiency as preservatives of steel, are : Zinc and barium chromate; zinc and lead chro mate; zinc oxide; zinc-lead white; barium chromate; ultramarine blue; blue chrome green; Prussian blue; lithopone, and willow charcoal. At the other end of the scale, and apparently aiding in corrosive activity, are lamp black, the most disastrous of all; blanc fix6; carbon black; the ochres; graphite; barium sulphate; bright red oxide and mineral black. Occupying a middle ground are the ordinary white leads; red lead; Venetian red, and the brown so-called `metallic paints." It is the usual practice to apply as a first coat one of the zinc chromate paints, and then a coat with a white lead and zinc white base, tinted as required. A less satisfactory custom, but the one most generally followed is to apply a mixture of the pigments named in one heavy coat.
Where three or more coats are to be ap plied and it is not practicable to use more than two kinds of paint, one can use for middle coat mixtures of topping and bottoming coats. Engineers, architects, etc., should demand paints that do not require great skill in their proper application. The requirements of an anti-cor rosive metal covering are, that it (1) Should hide the surface; (2) should cement itself to gether, and also cement itself to either damp or dry metallic surfaces; (3) should expand and contract under changes of temperature without breaking its own body; (4) should pre sent a hard, yet tough, outer surface; (5) should be impervious to water, marsh-gas or other gases; (6) should be unaffected by sun shine, heat, frost, dew or climatic changes; (7) should be unaffected by ordinary mechanical abrasion; (8) should wear evenly; (9) should fail by gradual wear, not by disintegration; (10) should leave a good surface for repaint ing; (11) should not require an unreasonable amount of skill in application; (12) should be homogeneous; (13) should dry fast enough; (14) should not be readily ignited; (15) should have power to absorb moisture or dampness from the metal.