About 18 months ago, we built a deck and a front porch on an old farm style houses here on Long Island, NY. The job looked beautiful, the clients were very happy.
The posts for the railings are made of CCA 4x4s, wrapped with “trimboard”. Trimboard is a 1x finger joint, factory primed pine board, which we use for all kinds of exterior trim applications, with no problems. The posts were painted with acrylic house paint. After last winter, the wrapping on some of the posts started showing some mold – real black patches of almost soot like stuff. About a month ago, the painter cleaned and repainted the posts, but now the patches are starting to come back! The posts wrappings are capped at the top and have a 3″ base at the bottom (both cedar). The wrapping does not touch the decking – it’s about 1/4″ off the deck.No other piece of trim we used (same material) exhibits this behaviour – only the posts.
What is causing the mold? How can I stop it? Any suggestion will be greatly appreciated.
Kroozer
Replies
I had this same problem 3 years ago with some pine material that was shipped to me here in the Puget Sound from upstate New York. At the time I made a joke and called it the New York Tree virus, but his makes me wonder. I installed 12" pine panelling horizontally, inside a dry, dehumidified, warm house and within two weeks these large black, sooty spots were coming out of the wood. Our first attempt at cleaning it up was with a product called Bio-wash. It did nothing. After allowing the Biowash process to dry, we sanded the affected areas, then belt sanded the whole board, and then with 120 grit on the orbitals. Then, and this is the part some people have a problem with, but it worked. We made a strong bleach and warm water mix, probably like 1/2 cup to the gallon, using different hardness of brushes applied the bleach mix. Using harder bristled brushes on the black spots, softer on the rest of the board. After treating an entire wall we let it dry at 68-72 degrees with a fan for air circulation for 2 days. There were some small grayish spots that were visible after applying clear finish, but the customers loved it's "character".
In your case I think you could pull the trim boards, strip the paint, and try bleaching. Then before you reapply the trim use a product like Kilz, or Zinnser 123 sealer/primer on both the CCA 4x material and all sides of the trimboard material. Re-apply the trim and paint.
Good Luck.
Sounds like a bad case of mildew in a way, but I'm no mold or fungus expert and won't pretend to be one on TV either.
Nonetheless, I don't think you have much to lose by applying a strong household bleach solution just as things are and see what results. I'd allow it to sit in contact for twenty minutes or so before rinsing well.
Maybe make sure the next paint you apply has a fungicide additive. Might do the trick.