We've used oxalic acid to remove the black, which I think is either the result of algae or fungus but I'm not positive. However if the black wood goes deep enough we found that oxalic acid (or anything else we tried) would not remove it.
This was almost always the case when the "black" had penetrated deep into a vein of wood. The only cure, we were told by a shipwright, is to sand it out, which in the case of wood trim or some other teak component will result in a dip in the wood, maybe a deep dip. So we did the best we could with the acid and then just left the remaining black in place and finished over it. Since it was in a vein all it does is make the vein more obvious. As our boat was made way back in '73 the Burmese old-growth teak that was used throughout the boat is highly figured anyway, so these few black streaks in the cap rail simply look like part of the wood's figuring.
But we found that oxalic acid was the best thing to remove surface blackening.* It seems to be the most recommended way to remove stains from teak on the Grand Banks owners forum, and since GBs, particularly the older ones like ours tend to have a rainforest of exterior teak on them, the GB owners tend to have a lot of experience dealing with it.
if you use it make sure to follow the mixing directions and have a hose handy to wash it off the gelcoat should it get on it.* The acid will also lighten the wood, so you either have to do all of it or, once the black has been removed as much as possible, stain that portion of the teak that has been lightened back to match the rest of it.
-- Edited by Marin on Monday 20th of December 2010 01:55:59 PM
