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, (7) 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
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
- an oil primer may be applied followed by an oil-type top
coat, preferably by the same manufacturer; or
- 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.
(7) If the top coat is latex paint (when viewed by the naked
eye or preferably, with a magnifying glass, it looks like a series
of tiny craters) it may either be repainted with new latex
paint or with oil paint. Normal surface preparation should precede