I recently made a door for a storage compartment on a boat. I made it from white oak and finished it with 2 coats of spar varnish and wax. After about a month of exposure on the boat, some black spots started developing under the finish. I assume it’s mold, but I don’t know for sure.
Has anyone else experienced this? How can it be prevented?
Thanks, Dave
Replies
Strip and sand the door. If needed, use oxalic acide to remove the black spots (available at paint & hardware stores).
The finish is not doing it's job well enough. Use a good brand of exterior varnish (boat repair shops carry some good brands) and apply 6-8 coats. Skip the wax.
Consumer reports has some info on tests they conduct on exterior finishes - take a look at your local library - some are MUCH better than others.
Paul
F'burg, VA
Thanks for the info. Guess I short changed the coats of finishes
Dave, the bottom line is that white oak is not a good wood for marine use. While it will not rot, its high tannin content means that water containing minerals--particularly iron--will cause it to discolor. Use a good marine grade exterior varnish applied with the recommended number of coats can work. Most marine grade finishes will recommend at least 5-6 coats. In addition, every little ding must be immediately repaired as water will now penetrate the finish and discolor and cause the finish to flake.
True marine finishes are available at marinas or boating supply stores. The brands to look for are Interlux, Woolsey, Pettit or Epifanes. Be sure to follow directions as to surface preparation and number of coats.
Thanks for your reply. You said white oak wasn't a good choice of material. In your opinion what should I have used. The original cover was done in white oak. In fact, I just duplicated what was there, but the original was about 17 years old, so there was no finish to compare to. Maybe I should have left it unfinished??
Dave
For areas exposed to the elements, mahogony and teak are best. White oak will certainly last, but it is always subject to discoloration from water that contains minerals.
Edited 8/26/2002 11:34:30 AM ET by Howie
Thanks for the education. And by the way, thanks to all Knots contributors. By the time I die, I'll be one smart sucker.
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