These volatile oils take a gaseous form at different temperatures, lie
partly dormant until the thermometer hovers at 90 deg. F. in the shade,
when they develop into gas, forming blisters in airtight paint, or
escape unnoticed in porous paint. This is the reason why coal-tar
paint is so liable to blister in hot weather; an elastic, soft
coal-tar covering holds part of its volatile oil confined until heated
to generate into gas; a few drops only of such oil is sufficient to
spoil the best painted work, and worse, when it has been applied in
priming, it settles into the pores of the wood, needing often from two
to three repetitions of scraping and repainting before the evil is
overcome. Now, inasmuch as soft drying paint is unfit to answer the
purpose, it is equally as bad when paint too hard or brittle has been
used, that does not expand and contract in harmony with the painted
article, causing the paint to crack and peel off, which is always the
case when either oil or varnish has been too sparingly and turpentine
too freely used.
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