The surface condition of the wood to which the finish is to be applied
can substantially affect the performance and hence the life expectancy of
the finish. The following procedures should prove helpful:
New wood such as cedar siding and trim, should be protected from
the weather before, during and after construction. It is seldom necessary
to carry out extensive surface preparation providing the wood has not
weathered for more than two weeks and is clean and dry. If it has been
contaminated by dirt, oil and other foreign substances they must be
removed.
For smooth-planed, flat-grained cedar, some surface preparation may be
desirable. On flat-grained wood, the surface should be scuff-sanded with
50-60 grit sandpaper. This procedure will greatly increase the coatings
performance but will not detract from a smooth finish. Surface preparation
is not necessary for textured cedar.
Weathered new wood that has been exposed to the elements for
longer than 2 weeks may have a degraded surface that is unsuitable for
painting. Preparing the surface by sanding, brushing, and washing before
applying the finish is recommended.
Paint finishes must be removed if the old surface is severely
peeled, blistered, or if cross-grain checking has occurred because of
excessive paint build-up. The removal of a film-forming finish is also
necessary if a penetrating stain or water-repellent finish is to be
applied to a previously painted or solid-color stained surface. Note that
changing from a film-forming to a penetrating finish sometimes does not
give satisfactory results because residual paint inhibits absorption.
Finishes can be removed by sanding, wet sandblasting, pressurized water
spray, electrically heated paint removers and chemicals. Although quick
and easy, sandblasting and pressurized water spray are not recommended
unless extreme care is taken to avoid damage to the wood's surface.
Special precautions to ensure worker safety must be taken if the old paint
is of the lead-based type.
Weathered water-repellent preservative finishes should be
cleaned with a non-ferrous bristle brush to remove loose fibers and dirt.
If the surface is soiled, it may be scrubbed with a mild detergent
solution. If mildew is present, it should be controlled. The surface
should be thoroughly rinsed and allowed to dry completely before
refinishing.
Caution: Never mix bleach with detergent containing ammonia as
the fumes can be harmful or fatal.
Weathered penetrating stains on mildew-free surfaces are
relatively easy to refinish. Excessive scraping or sanding is not
required. A stiff, non-ferrous bristle brush may be used to remove surface
dirt, dust and loose wood fibers before applying the stain.