Boiled Oil. The mistake is sometimes made of using boiled linseed oil for exterior house painting, under the impression that boiled oil is better than raw oil. In the first place, a great deal of the oil which is sold as boiled oil, has never been boiled at all, but is simply raw oil overloaded with driers. Such oil is known to the trade as "bunghole-boiled;" and while it dries quickly, it causes paint thinned with it to perish rapidly, because it hastens unduly the oxi dation of the oil. Sometimes this "bunghole boiled" oil has been heated for a short time in a steam-jacketed kettle, but this does not make it real boiled oil. Genuine boiled oil is actually boiled, which drives off a portion of the moisture present in the oil; and lead or manganese driers are added to the oil to hasten its drying. The result is a heavy-bodied, partially oxidized oil. Paint mixed with it dries rapidly on the sur face, but the under part of the paint film remains soft and tacky for a long time. When the paint appears to be dry to the touch, the mistake is frequently made of putting on the final coat, which exerts a pulling influence on the soft un dercoat as the upper coat hardens, causing it to shrivel up or to wrinkle like an alligator's hide. If such a paint surface is exposed to the direct rays of the sun, the heat causes the soft, undried paint nearest the wood to swell up, and the re sult is a blistered surface. In nine cases out of ten, where boiled oil is used, it can be confidently predicted that this blistering will occur, for it is almost impossible to brush out a paint mixed with boiled oil sufficiently to avoid this undried layer of paint beneath the outer hardened skin.
Blistering. By far the most frequent cause of blistering, especially when it is found only in occasional places or over a limited portion of an otherwise intact paint surface, is the presence of moisture back of the paint; and one of the first things for the painter to look for when called to account for the presence of these blisters is a leaky gutter, or perhaps some moulding or win dow-cap over which the rain may be driven by the wind to find its way into the spaces behind the boards. Often the leak will be at some dis tance from the blisters, because the water will follow along a rafter or a stud until it meets with some darn or obstruction, when it will soak into the boards, finally being drawn out by the sun's heat to cause blisters on the paint.
With the point of a knife blade, prick a bubble caused by moisture, and it will usually be found' to be full of water. Sometimes these bubbles will disappear after a few weeks or months, the moisture either evaporating through the paint or being absorbed by the wood; and if the leak or other cause of moisture be removed, the paint will become solid and there will be no further trouble, especially if the paint pigments have been of an elastic nature, such as white lead or the various iron oxide or earth pigments.
A very fruitful cause of blistered paint is where the plastering has been done after the painting, more especially if the window sash have been put in, or the windows have been cov ered with muslin and fires used to dry the plas ter. The heat will frequently drive the moisture across the openings between the studs and through the outside boards, causing blisters in the paint.
Where good materials are used, where there is sufficient turpentine•to harden the paint film in the priming coat, and where the paint is well brushed out in thin coats, with ample time—not less than a week—allowed between the applica tions of the coats of paint, there is practically no danger of blistering, unless it be due to some cause which the painter is powerless to prevent, such as moisture finding its way behind the paint from a leak or from freshly plastered walls.
When a painted surface has blistered badly, it should be thoroughly scraped off, and all blis ters sandpapered so as to give smooth edges before repainting. It sometimes happens that this will not make a solid job, and it is then nec essary to burn off the oil paint. While this is expensive, it is the most effective remedy for blistering, since the heat of the torch will draw out any moisture left in the wood, and will also remove sap, to the presence of which blistering is sometimes due.
Painting a Weather-Beaten Surface. It does not always follow that boards which are wet or damp when painted will afterward blister. Indeed there are certain occasions when it may prove advantageous to wet down the boards be fore painting. This is when it is desired to paint an old, weather-beaten building, especially one that has never been painted before, and the woodwork of which has become almost like a sponge in its capacity for soaking up moisture. Such a surface will suck up the oil from the priming coat of paint, leaving the pigment as a dry powder on the wood, even though it is made much more oily than usual. It has been found by experienced painters that by wetting down such a surface with water, applied freely with a whitewash brush, just long enough before painting to allow the water to soak into the wood and swell it somewhat, an oily priming coat, with rather more drier than usual, may be safely applied, and it will not only take less paint than if it were applied to the bare wood, but the paint will actually hold better. The water is quickly absorbed into the wood, and has no chance to cause blisters. In this case, the wetting down practically takes the place of a surfacer, filling the pores of the wood and preventing the paint from sinking too deeply into them. The added driers serve to harden the oil before the mois ture has been absorbed sufficiently to permit the oil to follow it; and thus the paint film remains intact on the surface, instead of being denuded of its oil and left as a mere powder.