Most of the screw holes penetrating the teak and holly sole of my boat are surrounded by black stains that appear to be under the polyurethane finish. I've tried soaking the affected areas with bleach and oxalic acid, but to no avail. I also tried sanding out the stains, but they go too deep into the wood. Do you have any ideas?
Tom Wood responds:
You don't say whether the screw heads are covered by bungs, so I'm assuming that they are not. The main reason, in fact, that high-quality boat joiners counterbore for screws and cover the heads with plugs is to prevent water intrusion into the grain of the wood. A bung dipped in varnish before it is tapped into place effectively seals the area. An exposed screw can actually draw moisture into the grain of the wood as it heats and cools, causing condensation.
I also am presuming that the teak and holly sole to which you refer is really a piece of plywood with a thin veneer on top. It is very difficult to sand this without abrading through the teak veneer into the unsightly underlayment. The plywood also is more prone to water intrusion around the screw holes, which always discolors the area in time. Wood bleach comes in different forms and bears no relationship to the Clorox bought at the grocery store. Oxalic acid is fairly mild, but it is good if mixed into a paste and allowed to stand for a fair amount of time. But the screw and the varnish must be removed first so the bleach can get to the stained wood.
The only other reasonably quick remedy is to remove the screws, sand off the finish around the holes, and apply a stronger bleach such as two-part Te-ka, making sure to work it down into the hole. This stuff is strong and requires care to avoid damage to fiberglass, cloth, eyes, and skin. Follow the directions and use the second-part neutralizer before rinsing. The area must be completely dry before proceeding. If the stain is gone, apply a coat of 3M Marine Teak/Wood Seam Sealant around the inside of the screw hole to prevent a recurrence.
If this process doesn't do the trick, the next step is to determine the thickness of the wood. If the ply is at least 3/8-inch thick, it is possible to remove the screws, cut the stained wood away with a countersink and a half-inch or 5/8- inch counterbore for a plug, replacing the screw with a flat head of the same size and putting a proper bung over the screw head. The plugs will need to be chiseled, planed and sanded flush before refinishing.
If the wood is too thin to be plugged, you have only two choices left. Replacing the entire sole is one and ignoring the problem is the other. Good luck.