Q: I have a home with three large decks. I built the decks two years ago using Port Orford cedar and finished them with two coats of Cetol marine varnish. One deck gets moderate sun in the middle of the day and is shaded the rest of the time. Part of the deck is under an overhang and gets no sun.
Last year I started seeing white material on the knots in the wood. When the deck is wet, they become invisible. It is only the knots that are in the sun, not the section under the overhang. The white material looks sort of like powder, but does not scrape off.
I talked to the folks at the lumberyard where I bought the wood and the deck finish; they think it is resin that is coming from the wood and suggested that I sand it down and refinish it. I’m not quite sure how to sand down the knots as they are actually below the surface of the deck.
I’m game to do the sanding but obviously would like to explore alternatives.
A: The guys at the lumberyard are probably right. It sounds as if you have a little resin bleed-through from the knots in the decking. The spar varnish sealed it in. But the varnish does not affect expansion and contraction of the wood (and the knots). Direct sunlight will enhance the expansion/contraction process.
The resin is also drying in the sunlight. This accounts for the white film you are seeing. Think of pine or fir logs you’ve seen where the pitch has seeped out of the log and has dried into opaque white rivulets.
We also agree that sanding is probably the best way to try to alleviate this problem. Sanding the knots shouldn’t be too tough a job. It probably is worth investing in a small orbital power sander. These are available at home centers and hardware stores for less than $100. Make sure to use medium- to fine-grit sandpaper.
There’s one further step we’d suggest you take after you get the knots sanded and before you apply another coat of spar varnish. Take some lacquer thinner or acetone and wipe down each knot thoroughly before you refinish. This should help remove any resin residue left after sanding.
Also, when you apply the finish, be sure to clean the surface of dust and debris and feather the new material into the existing finish. Lightly sand the existing finish 1 to 2 feet on either side of the knot you have exposed. This provides a rough surface, known as a “tooth,” that enables the new material to adhere to the existing finished surface. Apply the new varnish fully over the knot and brush it out over the roughened surface so that it blends with the existing finish. This should restore the look of what we can only imagine is your beautiful Port Orford cedar deck.
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