excessive wetting, which tends to raise the grain. Wash
a small area at a time, then rinse and dry it immediately.
Wood that is to receive a natural finish (meaning
not concealed by an opaque coating) may require
bleaching to a uniform or light color. To bleach, apply
a solution of 1 pound of oxalic acid to 1 gallon of hot
water. More than one application may be required. After
the solution has dried, smooth the surface with fine
Rough wood surfaces must be sanded smooth for
painting. Mechanical sanders are used for large areas,
hand sanding for small areas. For hand sanding, you
should wrap sandpaper around a rubber, wood, or metal
sanding block. For a very rough surface, start with a
coarse paper, about No. 2 or 2 1/2. Follow this with a
No, 1/2, No. 1, or No. 1 1/2. You should finish with
about a No. 2/0 grit. For fine work, such as furniture
sanding, you should finish with a freer grit.
Sap or resin in wood can stain through a coat, or
even several coats, of paint. Remove sap or resin by
scraping or sanding. Knots in resinous wood should be
treated with knot sealer.
Green lumber contains a considerable amount of
water, most of which must be removed before use. This
not only prevents shrinkage after installation, but pre-
vents blistering, cracking, and loss of adhesion after
applied paint. Be sure all lumber used has been properly
dried and kept dry before painting.
Conditioners are often applied on masonry to seal
a chalky surface to improve adhesion of water-based
Table 8-1.Treatments of Various Substrates