The question of what undercoat to apply for some specific painting job is one that generally bothers the amateur painter, but the selection of the proper priming coat is little more than following a few common sense rules.

For all general purposes undercoats can be divided into four classes: for use under enamel, varnish, lacquer, and flat wall paint.

In enameling, the procedure is to build up a foundation of flat undercoats before the final finishing coat or coats of full enamel is applied. The process is substantially the same regardless of the surfaces being finished. However, where new woods of the open-grain type are to be finished, the pores should be filled with paste filler. This also should be done on previously finished open-grain woods from which the old coatings have been removed with paint and varnish remover.

The material generally used for enamel undercoats is a prepared "enamel undercoater," sold at most paint stores for the purpose. White lead and oil paint is also used, but the home craftsman usually avoids mixing his own paint. Flat wall paint also makes a good enamel undercoat. While it is softer than the specially prepared undercoaters and therefore does not make quite so hard and firm a foundation, it has the advantage of coming in a variety of colors.

The prepared undercoaters generally come in white only and are adapted for use under white enamel and the light tints of ivory, cream, and gray. The amateur painter will find it better, when enameling in stronger and brighter colors, to use flat wall paint of a color closely approximating the enamel finish.

Any skipped or too thinly spread-out places will not be so apparent when there is Dot so great a contrast between the undercoating and the finishing coats.

The surface should be rubbed down with fine sandpaper (No. 00) to platelike Smoothness before the first coat, and lightly between coats, to remove brush marks and level off nibs, bits of grit, and dust. The loose particles must be dusted off thoroughly.

Just a word about the application of enamel undercoats. A three-coat job is generally used for white and the light tints, as follows: first coat, flat undercoater; second coat, a mixture of equal parts of flat undercoater and enamel; third coat, full enamel.

Yellow pine, cedar, cypress, and similar pitchy, resinous, or oily woods, should be sealed over with a coat or two of thinned shellac before starting to build up the enamel foundation, as it prevents the pitch or oily substance from coming through and discoloring the finish.

In the refinishing of mahogany and other dark, stain-finished, woods with light colored enamels, a sealing coat of shellac should be applied before the first undercoats; this will prevent the stain from bleeding through in the majority of cases. It is, however, almost impossible to apply a light enamel finish satisfactorily over some types of penetrating red mahogany and cherry stains.

The most practical way to build up a varnish foundation, especially for the home finisher, is to apply as many coats of varnish as necessary over a foundation provided by the use of paste filler, if the wood is of the open-grain type.

The filler, which ordinarily comes in paste form either in a light or "natural" color, or stained dark, is reduced with benzine or turpentine to about the consistency of heavy cream, and is applied to the surface with a brush. After standing a few minutes until it commences to set - this is indicated by a dulling out or loss of gloss - it should be vigorously wiped off with a cloth, across the grain. Care should be taken that every bit of the filler is removed except that which has entered the pores. Allow the work to stand for at least twenty-four hours, until the filler in the pores has dried. The surface is now ready for the finishing coats of varnish.

Liquid fillers are used to some extent on close-grain woods, where paste filler cannot be forced into the pores. Although it is a general practice in the varnish finishing of interior woodwork and floors of close-grain woods to start applying the varnish directly over the wood without any undercoat material of any kind, a liquid filler may be used to advantage on furniture and other surfaces where the finest finish is desired.

Some finishers also apply a coat of liquid filler to open-grain woods after the paste filler has dried thoroughly hard, to fill the tiny wood cells that are not filled by the paste filler, thus giving an absolutely smooth surface and permitting a finish of mirrorlike appearance. Liquid filler is applied with a brush and after it is thoroughly dry is rubbed down close to the wood with fine sandpaper or steel wool.

A brushing lacquer, which dries a few minutes after application, is somewhat different from other finishing materials. It is reasonably satisfactory without undercoats. In the first place, though it does not have any wood-filling properties to speak of, its make-up is such that it seals over the surface, very much as shellac, instead of soaking into it as do painting materials made with oil and turpentine. Therefore it will stand out on the surface fairly well without the use of an undercoating. It also has better hiding power than the transparent enamels; hence, surface discolorations are usually hidden and a solid covering finish obtained with two coats of lacquer without the use of undercoats.

For the finest possible lacquer finish on new work, however, open-grain woods should be filled with paste filler (as previously described for varnish undercoats). Close-grain woods also may be brought to a better finish if liquid filler is employed to fill the small pores.

A practice favored by many is to apply a wash coat of thinned shellac (regular four-pound cut shellac reduced with about an equal part of denatured alcohol) as a primary coat. Since lacquer can be applied perfectly over shellac, this method may be regarded as good general practice for the amateur finisher. Of course, the advantage of this sealer coat is much greater in the case of the softer woods, and with the extremely soft woods the use of shellac is almost necessary for satisfactory results, unless several extra coats of lacquer are applied.

The use of shellac as a first coater in refinishing old painted, varnished, enameled, and stained surfaces with brushing lacquer also renders the use of the lacquer more satisfactory.

Prepared undercoaters for use under lacquer are sold by some manufacturers. They combine the qualities of a sealer with a higher solid content than the lacquer itself, thus adding fullness and richness to the finish. These should be used according to the directions accompanying the particular make of under-coater that is being used.

In conclusion, a word should be said about undercoats on interior walls. It is absolutely necessary that bare plaster walls which have never been previously painted be given a sizing coat to seal over the extremely porous plaster. If this is not done, an indefinite number of coats could be applied to the surface, soaking in as fast as applied, without producing a satisfactory finish.

Regular wall size or varnish size (sold at all paint stores), mixed with equal parts of the wall paint being used for the work, is extensively employed for the sizing coat. Prepared wall primers also are now available in most localities; in these the size is already incorporated, making a very convenient form of material to use. Either type of material is thoroughly satisfactory.