by GreenLake » Sun Nov 25, 2012 6:56 pm
I've tried exterior PU (local brand) on bare wood and have had mixed to bad experiences with it, particularly on the floor boards, but also on coamings.
The PU would crack and blister around the screw holes and in many other places after no more than a season. (Boat under cover when not in use).
So, I took it all down to bare wood, using heat guns, scrapers and orbital sanders (starting at 36 grit).
Then I coated the pieces with System Three ClearCoat and/or their laminating epoxy- Silvertip. (They both work, the clear coat levels a bit better and gives a thinner coat, so you need more coats). I placed all the pieces flat onto a nail bed.
The nail bed is just a strip of wood with nails hammered in so they stick up. I used three of these, because the rear coaming is shorter, so I could support the longer pieces at both ends and in the middle, and the shorter on on both ends. I set it up so one nail lined up with one of the screw holes in the boards, and left that one taller so it would hold the boards in position. That setup kept the boards raised up, so there wasn't any issue with epoxy dripping and pooling on the underside, and I could even finish both sides in one go. Yes, where the nails rest there's a pinprick size defect, but I made sure it was on the back side.
After the epoxy had cured, I used System Threes Water-reducible Linear Polyurethane and gave things as many coats as I had patience for.
One thing that I would do differently is that I would probably do differently is to apply a bit of a stain to hide color variations, possibly even use a bit of oxalic acid to deal with any darkening. I tried that when I restored my tiller and it really took care of places where the wood had darkened under cracks. The point is to get an even color. You can wipe the wood with alcohol to see what it would look like with a varnish. I didn't do anything like that for the coamings and there are some color variations as a result. Given the age of the boat they tend to look "distinctive". Most traditional varnishes have a little bit of a yellowish or reddish tint to them, and that's something else that you can emulate with a stain, because the PU will be clear.
If you do use epoxy - measure precisely and mix well! If you get bubbles you can "pop" them by flicking a propane torch over the coated piece, adding just enough heat for the bubble to expand and burst, no more. Also, if you do use epoxy, you need to coat all sides (and the insides of all screw holes) because if any water wicks in, it won't be able to come out again.
The PU coating (especially if you use the catalyst that comes with it) will be very touch and it benefits from the fact that the epoxy will prevent the wood from "working". But you do need to keep it intact, because epoxy is sensitive to UV degradation.
Alternatives: regular varnishes will not work well, they tend to not hold up and get rubbed off the exposed edges. If you love running around with a brush in your hand and the boat is close by, then oiling the coamings is an alternative. Oil soaks in and therefore doesn't rub off, but needs to be touched up several times each season. Luckily the prep required is also minimal.
~ green ~ lake ~ ~