This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
The following, if carefully carried out, gives the best satisfaction: The first step consists in thoroughly cleaning the surface of the iron, removing all adhesions in the way of dirt, rust, etc., before the question of priming is considered. As paint in this instance is applied more with a view of protecting the iron from atmospheric influences, rather than for a decorative effect, careful attention should be devoted for securing a base or surface which is calculated to produce a thorough and permanent application. A great deal depends upon the nature of the metal to be painted. Common cast iron, for instance, possessing a rough exterior, with ordinary precautions can be more readily painted with the prospect of a permanent adhesion of the paint, than a planed steel or wrought-iron surface. With the latter it has been demonstrated that a hard and elastic paint is needed, while with regard to cast iron, other paints containing iron oxides are more suitable. For good drying and covering properties, as well as elasticity, a good boiled oil to which has been added an adequate proportion of red lead will be found to form an excellent paint for smooth metal surfaces. The primary object is to protect the surface of the iron from moisture for the purpose of avoiding rust. The priming must therefore be carried out so that it will stick, after which subsequent coats may be added if desired.
It is advisable that articles made of iron should first be coated with linseed-oil varnish. It dries slowly, hardens, and enables the operator afterwards to exercise an effective control over the condition of his material. Iron must be absolutely dry and free from rust when it is to be painted. It is best to apply next a coating of hot linseed oil; when dry this should be followed by a priming of pure red lead in good linseed oil, and the iron should then be painted as desired, using ground oil paints and leaving an interval of a week between each coating. Cementing should be done after the red lead priming, but the last coat must not be given until the whole is thoroughly dry. Bright oil paints and an upper coating with plenty of oil resist the effects of heat better than thin coatings; moreover, rust can be detected in its early stages with the former. Coatings of tar and asphalt (asphalt dissolved in turpentine) are practicable for underground pipes, but are not adapted for pipes exposed to the air, as they are quickly spoiled. Asphalt varnish, used for coating coal scuttles, fire screens, etc., consists of asphalt dissolved in linseed-oil varnish. Iron stoves and stovepipes are best coated with graphite.