 Decorative features were painted with a traditional oil-based paint as a part of the rehabilitation. Photo: NPS files. |
Based on the assumption that the exterior wood has been painted
with oil paint many times in the past and the existing top coat
is therefore also an oil paint, it is recommended that for
CLASS I and CLASS II paint surface conditions, a top coat of high
quality oil paint be applied when repainting. The reason for recommending
oil rather than latex paints is that a coat of latex paint applied
directly over old oil paint is more apt to fail. The considerations
are twofold. First, because oil paints continue to harden with
age, the old surface is sensitive to the added stress of shrinkage
which occurs as a new coat of paint dries. Oil paints shrink less
upon drying than latex paints and thus do not have as great a
tendency to pull the old paint loose. Second, when exterior oil
paints age, the binder releases pigment particles, causing a chalky
surface. Although for best results, the chalk (or dirt, etc.)
should always be cleaned off prior to repainting, a coat of new
oil paint is more able to penetrate a chalky residue and adhere
than is latex paint. Therefore, unless it is possible to thoroughly
clean a heavily chalked surface, oil paints--on balance--give
better adhesion.
If however, a latex top coat is going to be applied over several
layers of old oil paint, an oil primer should be applied first
(the oil primer creates a flat, porous surface to which the latex
can adhere). After the primer has thoroughly dried, a latex top
coat may be applied. In the long run, changing paint types is
more time consuming and expensive. An application of a new oil-type
top coat on the old oil paint is, thus, the preferred course of
action.
If CLASS III conditions have necessitated total paint removal,
there are two options, both of which assure protection of the
exterior wood: (1) an oil primer may be applied followed by an
oil-type top coat, preferably by the same manufacturer; or (2)
an oil primer may be applied followed by a latex top coat, again
using the same brand of paint. It should also be noted that primers
were never intended to withstand the effects of weathering; therefore,
the top coat should be applied as soon as possible after the primer
has dried.
The recommendations outlined in this Brief are cautious because
at present there is no completely safe and effective method of
removing old paint from exterior woodwork. This has necessarily
eliminated descriptions of several methods still in a developmental
or experimental stage, which can therefore neither be recommended
nor precluded from future recommendation. With the ever-increasing
number of buildings being rehabilitated, however, paint removal
technology should be stimulated and, in consequence, existing
methods refined and new methods developed which will respect both
the historic wood and the health and safety of the operator.
"Exterior Paint Problems" an Historic Preservation Brief
