I’ve just power-washed the black spots off the carport soffit and revarnished the area. When the job was finished, I still had black spots, plus splits in the surface of the wood. What should I have done before applying the new varnish?

First, take a utility knife and scrape a few black spots in different areas of the soffit, scraping down to the bare wood. If there are no black spots left in the wood once the spots are removed, then the spots are embedded in the surface of the varnish only.

The spots are mildew or mold, part of the fungi group. Unlike plants, fungi do not produce their own food so they must derive their nutrients from the animals, plants, or decaying matter on which they live. In this case, the varnish is probably over ten years old and can no longer resist the fungi. Early varnishes were made of natural fossil, animal, and plant resins on which the fungi feed, which explains why black spots appeared in or on the surface of the varnish.

Second, I do not recommend power-washing to remove black spots because it forces moisture into and behind the soffit material, encouraging fungal growth. If the new finish is applied to the soffit before it completely dries, then small splits, about 2 inches long, may appear in the surface, parallel with the wood grain. If you decide to power-wash, be advised that two or three days is not long enough for any moisture trapped behind the finished surface to dry.

THE SOLUTION

1. Using varnish remover, strip the soffit down to the bare wood.

2. Apply a mixture of 4 oz. (1 cup) TSP (trisodium phosphate), 1 quart liquid laundry bleach, and 3 quarts of warm water to the soffit surface. Best results are achieved by scrubbing with the grain of the wood using a medium brush. When the surface is clean, rinse with fresh water from the garden hose. If TSP is not available in your area or its use is prohibited because of ground water regulations, your paint dealer can suggest an environmentally safe product.

3. Let the area dry for at least one week, depending on the time of year and the humidity. Glue under pressure any splits in the soffit material using a waterproof wood glue. Then apply exterior sealer and two uniform coats of spar varnish.

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C.R.S., Inc. · Spokane, Washington · USA