This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
The novice in coach painting is quite as likely to get bewildered as to be aided by much of the information given about roughstuff, the more so as the methods differ so widely. One authority tells us to use a large proportion of lead ground in oil with the coarser pigment, while another says use dry lead and but a small percentage, and still another insists that lead must be tabooed altogether. There are withal a good many moss-grown superstitions associated with the subject. Not the least of these is the remarkably absorbent nature which the surface that has been roughstuffed and "scoured" is supposed to possess. By many this power of absorption is believed to be equal to swallowing up, not only all the color applied, but at least 3 coats of varnish, and none of these would think of applying a coat of color to a roughstuffed surface without first giving it a coat of liquid filler as a sort of sacrificial oblation in recognition of this absorbing propensity. Another authority on the subject has laid down the rule that in the process of scouring, the block of pumice stone must always be moved in one direction, presumably for the reason that some trace of the stone is likely to be visible after the surface is finished.
If the block of stone is scratching, perhaps the appearance of the finished panel may be less objectionable with the furrows in parallel lines than in what engravers call "cross-hatching," but if the rubbing is properly done it is not easy to discover what difference it could make whether the stone is moved in a straight line or a circle. As to absorption, it cannot be distinguished in the finished panel between the surface that was coated with liquid filler and that to which the color was applied directly, except that cracking always occurs much sooner in the former, and this will be found to be the case with surfaces that have been coated with liquid filler and finished without roughstuff. Among the pigments that may be used for rough-stuff, and there are half a dozen or more, any of which may be used with success, there is no doubt but that known as "English filler" is best, but it is not always to be had without delay and inconveniences.
Yellow ocher, Reno umber and Key stone filler are all suitable for roughstuff, the ocher having been used many years for the purpose, but, as already remarked, the English filler is best. This is the rule for mixing given by Nobles and Hoare: Four pounds filler, 1 pound ground white lead, 1 pint gold size, 1 pint varnish and 1 1/2 pints turpentine, or 3/4 pint good size and 1/2 pint boiled oil in lieu of the varnish. In regard to the use of white lead ground in oil, it makes the rubbing more laborious, increases the liability to scratching, and requires a much longer time to harden before the scouring can be done, without in any appreciable manner improving the quality of the surface when finished.
It may be remarked here that the addition of white lead, whether ground in oil or added dry to the coarser pigment, increases the labor of scouring just in proportion as it is used until sufficient may be used to render the scouring process impossible; hence, it follows that the mixing should be governed by the character of the job in hand. If the job is of a cheap class the use of very little or no lead at all is advisable, and the proportion of Japan and turpentine may also be increased, with the result that a fairly good surface may be obtained with much less labor than in the formula given.
The number of coats of filler required to effect the purpose in any given case must depend upon how well the builder has done his part of the work. If he has left the surface very uneven it follows, as a matter of course, that more coats will be required to make it level, and more of the roughstuff will remain after the leveling process than if the woodwork had been more perfectly done. While the merits of a system or method are not to be judged by its antiquity, there should be a good reason to justify the substitution of a new method for one that has given perfect satisfaction for generations and been used by the best coach painters who ever handled a brush.
A well-known writer on paints says that the effect of a varnish is usually attributed to the manner of its application and the quantity of thinners used for diluting the melted gums, with the prepared oils and the oxidizing agents used in its manufacture. While this has undoubtedly much to do with the successful application of varnish, there are other facts in this connection that should not be overlooked. For example, varnish is sometimes acted on by the breaking up, or the disintegration of the filling coats; which in turn is evidently acted on by the wood itself, according to its nature.
With the aid of the microscope in examining the component parts of wood a cellular tissue is observed which varies in form according to the species and the parts which are inspected. This cellular tissue is made up of small cavities called pores or cells, which are filled with a widely diversified matter and are covered with a hard and usually brittle substance called lignin.