excessive wetting, which tends to raise the grain. Washa small area at a time, then rinse and dry it immediately.Wood that is to receive a natural finish (meaningnot concealed by an opaque coating) may requirebleaching to a uniform or light color. To bleach, applya solution of 1 pound of oxalic acid to 1 gallon of hotwater. More than one application may be required. Afterthe solution has dried, smooth the surface with finesandpaper.Rough wood surfaces must be sanded smooth forpainting. Mechanical sanders are used for large areas,hand sanding for small areas. For hand sanding, youshould wrap sandpaper around a rubber, wood, or metalsanding block. For a very rough surface, start with acoarse paper, about No. 2 or 2 1/2. Follow this with aNo, 1/2, No. 1, or No. 1 1/2. You should finish withabout a No. 2/0 grit. For fine work, such as furnituresanding, you should finish with a freer grit.Sap or resin in wood can stain through a coat, oreven several coats, of paint. Remove sap or resin byscraping or sanding. Knots in resinous wood should betreated with knot sealer.Green lumber contains a considerable amount ofwater, most of which must be removed before use. Thisnot only prevents shrinkage after installation, but pre-vents blistering, cracking, and loss of adhesion afterapplied paint. Be sure all lumber used has been properlydried and kept dry before painting.CONDITIONERSConditioners are often applied on masonry to seala chalky surface to improve adhesion of water-basedTable 8-1.—Treatments of Various Substrates8-6
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