But before any varnishing can be done, and for that matter any painting, it is essential that the pores of the wood are filled, so that the surface to be varnished has no soft and absorbent places, but presents a hard and glossy body. Woods like oak, ash, and chestnut have such large pores that paste fillers are required to fill them in. These paste fillers consist of a solid part like pulverized quartz and a liquid part of a quick-drying varnish. It is rubbed over the surface of the wood and into the pores and permitted to set, when the excess is then wiped off with excelsior and, finally, felt. When the wood is stained with an oil stain, this filler may be colored to match.
Architects are often shown samples of the beautiful finishes which are possible with the use of this or that manufacturer's stains and varnishes, and supplied with specifications by which they are told they can secure these finishes, but much to their sorrow the results are not like the samples, and probably never will be. All of these samples are made under ideal conditions by the most careful experts. Laboratory conditions and regularity and first-class skill can produce finishes on a small sample board which could not possibly be reproduced in a building except at enormous costs. In the first place, there is always more or less dust blowing around in a newly constructed building, and not the greatest care is taken in it to provide the exact control of humidity and temperature required for drying varnishes. And, as every one knows, the men who do the painting are generally far from being the most skilful artisans of their trade. It, too, is a big temptation to put on one or two heavy coats of varnish instead of three or four thin coats, and there is not an expert living who can tell how many coats of varnish are on a piece of wood after the work is done. Unless the architect has observed each step of the application, he cannot deny, when the painter shows him the finished woodwork, that there are not as many coats of varnish on it as he required in his specifications. Yet time will tell the tale, but then it is too late.
However, the treatment of floors and stair treads is the worry of many an architect, although he ought to remember that in factories sheet steel is laid on the floors at the doorways, and even this wears through. Why should he be disheartened if after a year the stair treads and the patches of floors near the door-sills are scratched down to the wood through coats of varnish one-thousandth of an inch thick? Even the best varnish will break down under this abrasion, but only the best should be used. Cheap floor varnishes are not worth the labor of laying, and yet how many spend money on them. Some architects, and with good reasons, prefer finishing the floors with wax instead of varnish. As a base for this wax, a thin coat of varnish is excellent. Various manufacturers have different formulas for floor waxes, and they are more or less complex, but generally turpentine is the softening and drying material. The wax paste is rubbed into the floor and polished with weighted brushes - a tedious job. However, it is a job which any servant or housewife of ordinary intelligence can perform, so that whenever the floors become worn around the doors or the stair treads become shabby, the housekeeper is able to repair them easily, and there is no doubt that a waxed floor is more beautiful than a varnished one. But remember the slipping and sliding rugs on a wax floor and be sure to fasten them down.
When examined critically, paint is not much more than a varnish with a finely ground opaque powder, called the pigment, suspended in it. This pigment takes away the transparent qualities of the varnish and gives a definite color to the surface. Enamels actually do use varnishes as their vehicle or base, but ordinary paint uses linseed-oil, which acts much like a varnish, in that it has the property of becoming hard and elastic under the oxidizing effect of the air.
The exteriors of most houses are painted with white-lead or zinc-white pigments mixed with linseed-oil. Zinc makes a harder paint than white-lead, but it is best to mix the two pigments together in the proportion of one-third of zinc to two-thirds of white-lead.
In extensive investigations the U. S. Bureau of Standards suggests that much saving of money in paint would be made if white paint were abandoned altogether in favor of dark-colored pigments for exterior use. Horrible suggestions, but these are the facts in the case! White and light-tint paints invariably fail on the south side of a house, before the paint on the other side shows signs of deterioration. This is because the light of the sun breaks down the strength of the linseed-oil, which is the body of the paint film. For this reason dark pigments, which are more opaque, cut off the light and protect the oil film more than the lighter-colored pigments.
Another common cause of failure in exterior painting is the application of it to the wood during unseasonable weather, when the surface of the wood is wet. Paint will only properly adhere to a wood surface when it is free of any moisture.
Another one of the causes of failure of lead and zinc paints for exterior work suggested by some authorities is the use of volatile thinners like turpentine and benzine. They say that such thinners should not be permitted on the job, for they are a temptation to the painter. If raw linseed-oil is used, and it is necessary to shorten the time required for drying, some good drier should be added, say 5 per cent. This drier should be pale in color and free from rosin. Driers are usually made of oil combined with a good proportion of lead and a little of manganese.
White pine, Douglas fir, yellow pine, cypress, or any of these woods, usually contain some knots, which are sure to damage exterior white paint unless properly treated. These knots have a certain amount of pitch in them, which will penetrate through any oil paint and leave an ugly mark. They should be covered with shellac, which is not affected by the pitch. Shellac is a spirit varnish made from shellac resins dissolved in alcohol. The yellow shellac is the strongest, but the white is used where a light-colored paint is to be applied on top of it. The pitch which is so bad in knots is often distributed throughout the wood, as in Southern yellow pine, and this will often cause the paint to peel off. To prevent this to a certain extent, some specifications advise using benzol in the priming coat, in order to make the paint penetrate more deeply into the wood and get a better grip on the surface.
The priming coat of any painting job should either be pure linseed-oil or linseed-oil with very little pigment in it. Its purpose is to fill the pores of the wood before the other coats are applied, for if an ordinary thick coat of paint were applied to raw wood, the surface would draw so much oil out of the film of paint that most of the pigment would be left dry and unfastened upon the outside.
Only after the wood has been given the priming coat is it then time to putty up the nail holes and other defects, and not before, because the dry wood, as in the case of paint, will suck out the oil from the putty and leave it without anything to bind it together. The best putty for this work is made of linseed oil with enough white-lead in it to make a thick paste. The putty which is commonly used, however, is made of whiting or ground chalk mixed with linseed-oil. This is durable if real linseed-oil is used, but often some inferior adulterant is substituted.
After the holes are all puttied, the other coats of paint may be added. At least two good coats should be applied, and three coats give superior results. Plenty of time should be allowed between coats to permit thorough drying of the previous one.