Mildew, dirt, soot and other pollutants often appear as black dots or specks on the surface. To determine the correct treatment, apply a small amount of fresh household bleach to these black areas. If the dark discoloration lightens quickly, mildew is present. Dirt, soot and other pollutants will not change color but may simply move within the bleach solution.

Surface chalkiness must be removed with a detergent (sodium metasilicate) wash before staining.

Cedar bleed (tannic acid) is a reddish-brown discoloration that may make old stain applications appear blotchy or uneven. If you think your surface has cedar bleed, call Cabot's Technical Service and Support Department for assistance. An oxalic acid based solution will effectively remove the discoloration.

These dark vertical lines are caused when the heads of nails rust and water carries these brownish stains down the siding or shingles. Although difficult to remove, the effect can be minimized by cleaning. Apply Problem-Solver Wood Brightener to the entire area, leave on the surface for 15 minutes, and then rinse with water. If the area looks unsatisfactory, the only option may be to prime the nail heads prior to applying a solid-color finish.

First, identify and remove the source of moisture causing the problem. Coatings can be removed with Problem-Solver Wood Stripper. Apply wood stripper for oil-based coatings with a pump sprayer. Finally, scrape and sand the wood to sound condition. Be sure your surface is clean and thoroughly dry before staining. Use caution when scraping and using wood strippers.