The scale on iron or steel is the only serious trouble which the painter has to contend with. Rust can be removed or utilized with the oil, making a good paint, but unless time can be given it is better to remove the rust.
If possible let tanks get thoroughly rusted, then scrape off scale and rust with files sharpened to a chisel edge, rub down large surfaces with sandstone, and use No. 3 emery cloth between rivet heads, etc., then wash off with turpentine. This will give you a good solid surface to work upon.
For priming I use 100 pounds white lead (in oil), 10 pounds dry red lead, 13 pounds Prince's metallic, 8 quarts boiled oil, 2 quarts varnish, 6 quarts turpentine, and grind in the mill, as it mixes it thoroughly with less waste. I mix about 250 pounds at a time (put into kegs and draw off as wanted through faucets).
This o-le-ag-in-ous compound can be worked both ways, quickly by adding japan, slower by adding oil, and reduce to working consistency with turpentine.
Without the oil or japan it will dry hard on wrought iron in about seven days, on castings in about four days. When dry putty with white-lead putty, thinned with varnish and turpentine, and knifed in with a "broad-gauge" putty knife. Next day sandpaper and apply first coat rough-stuff, which is, equal parts, in bulk, white lead and "Reno's umber," mixed "stiff" with equal parts japan and rubbing varnish, and thin with turpentine. Next morning, second coat rough-stuff, made with Reno's umber, fine pumice stone, japan, and turpentine. At 1 o'clock P.M. put on guide coat for the benefit of the small boys, which is rough-stuff No. 2, darkened with lamp-black and very thin. The addition of fine pumice to rough-stuff No. 2 encourages the boys in rubbing, and prevents the blockstone from clogging.
By the time the last end of the tank is painted the first end is ready for rubbing, though it is better to stand until next day.
After rubbing sandpaper and put on very thin coat of varnish and turpentine (about equal parts). This soaks into the filling, hardening it and making a close, smooth, elastic surface, leaving no brush marks and being more durable than a quick-drying lead. This can be rubbed with fine sandpaper or hair to take off gloss, and colored the next morning, but it is better to remain 24 hours before coloring.
Upon this surface an "all japan color" would, before night, resemble a map of the war in Egypt, but by adding varnish and a very little raw oil to the "japan color," making it of the same nature as the under surface, will prevent cracking.
If I sandpaper in the morning, I put on first-coat color before noon. Second ditto afternoon, and varnish with rubbing varnish that night; rub down, stripe and letter next day, though I consider it better to stripe and letter on the color, and varnish with "wearing body varnish."
The tank is then ready for mounting. When mounted I paint trucks and woodwork, two coats lead, color, "color and varnish," and finish the whole with "wearing body varnish." Time, from 14 to 16 days.