Originally Posted by cnbirrell
I'm just finishing up building a new swim platform for my GB36 and am wondering what the thinking is on a finish. I made it out of a combination of teak and mahogany, to make use of a mahogany plank I had. Should I just let it weather or is there something I should put on it?
I would ask your question to the GB owners site Grand Banks Owner's Resources
, specifically to Bob Lowe on that forum.
Most of us let the teak swimstep weather gray. Ours is almost 40 years old now and while it's not a showpiece and I want to replace it someday to get a wider one it is still holding up surprisingly well.
The problem with putting any finish on a swimstep is that it gets wet a lot and it flexes a lot. Which means that lots of little cracks will start showing up in the finish, particularly at the joints, and moisture will soon find a way under the finish. When it does. the finish will separate and flake off in pretty short order. This applies as much to things like Cetol as it does to varnish, Bristol, etc. This is why almost all teak swimsteps are left unfinished.
The big question mark here is your use of a mahogany plank in the swimstep. Mahogany doesn't have near the rot-resistance of teak. This is why boats with mahogany planked hulls---- like GB woodies--- are painted on the outside and kept dry on the inside. But the Catch-22 is that if you paint or finish your swimstep, the finish won't stay on very long for the reasons I stated above. And once that mahogany gets wet, particularly down in the joints, there is a potential for a rot problem. GB owners know all about what happens when mahogany gets wet over a period of time because the window frames on most GBs are made of mahogany. Salt water will slow the development of rot but the boat will presumably be rained on, too.
Anyway, if it was me I'd post this question including the bit about the use of mahogany in the swimstep, to the GB forum and see what the experienced shipwrights like Bob Lowe have to say on the subject.