The methods adopted for the class of work will, of course, depend upon the quality of the finish required. Cheap bedsteads may be done by dipping in one coat of black japan and stoving at 350° F. In better class work several coats may be applied either by dipping or spraying, and each may be rubbed down after stoving. If a good enamel is used this is unnecessary.
When the work is white several coats--say, at least three--are necessary, for white japans cannot be made which will cover sufficiently in one coat. White lead should not be used in this class of work for the reason that a good deal of handling of parts is necessary, while the rubbing down is sometimes done with glass paper and cause dust, both of which may give rise to lead poisoning. All of the best white japans or enamels are made on a base of zinc oxide or lithopone (zinc sulphide), or a mixture of them.
This work is usually done by dipping in paint which dries "flat," i.e., without gloss. The colour is, as a rule, black. In fitting and adjusting the work in order that the parts will collapse readily it is necessary to apply a lubricating oil where the rods meet. This lubricating oil is difficult to remove after the adjustment has been completed, and therefore the last coat of paint is usually put on by hand. It is suggested, however, that raw linseed oil might be used instead of lubricating oil, and that this could be wiped off as soon as possible after the adjustment. In such a case the paint could be applied by dipping, as if even a little of the raw linseed oil were not removed it would not interfere with the drying of the paint to any material extent.
The process employed by a very large firm of American manufacturers is as follows:--
When the castings come from the machine shop they are boiled for 20 minutes in ammonia water to kill the grease (1 of strong ammonia to 20 of water), and are then rinsed in hot water. Any holes or inequalities in the castings are then stopped with a filler made from dry white lead, mixed into a stiff paste with some varnish, brown japan dryers and dry lampblack made quite stiff. When the stopping is hard the castings are dipped into a black metallic coating thinned with 62% benzine to a specific gravity 850. They are then dipped with two coats of finishing japan (sp. gr. 830) flatted with powdered pumice after each coat and leathered off. The parts which are not plated or flatted are given 5 hours stoving at 325° F., and are then rubbed with pumice and carpet pad until flat. Cut out gold transfers are then fixed with transfer varnish and the work is stoved at 185° F. The parts are then sponged and dusted, and are given a good coat of polishing varnish (sp. gr. 950) Finally the work is rubbed down fine with glass paper and pumice powder and then polished with machine oil and rottenstone powder for the iron, or rottenstone and benzine for the tables. The latter, as well as the covers for these, are not usually dipped but brush finished.