I have a problem with birch plywood that has a Minwax® stain and Minwax Helmsman® Spar Urethane finish. The problem I am encountering is the bubbling of the veneer and the splitting of the veneer after the wood was exposed to the rain. I would think that this problem would have happened after something would have contacted the plywood and scraped across the area, leaving scratches in the topcoat allowing moisture to get under the finish and causing it to bubble. To the best of my knowledge, nothing hit the wood to allow what my guess caused the problem. Here’s what the bubbled veneer looks like (see photo). I hope that it shows the problem to which I am referring.
Last year I asked a question about some water damage to the finish. I didn’t have the roof properly sealed and it allowed water to get to areas that were not covered by the urethane and water seeped under the finish. I removed the finish, re-stained and then refinished the affected areas and then sealed the roof and trim with clear silicone sealer. The fiberglass roofing material and aluminum trim overlapped the urethane so not to allow water to get above the finish and seep underneath. The sealer does not touch the urethane to damage the finish. It seals the trim to the roof and the screws holding the trim in place. After using the trailer several weeks ago, it was out in the rain again. All areas that were near the sealed area of the roof remained OK, but the afflicted area is just a little above the fender. I know I will have to completely strip the side of the trailer with the bubbled spot. What advice I need is how do I deal with the bubbled area? First, do I have to cut a veneer patch or is there a way to repair the veneer on this vertical surface? My next question, after re-reading eZine issue 272, where my water damage question was answered by Chris Marshall and Tim Inman: If the Minwax Helmsman wasn’t the best choice for finishing the trailer, what finish would have been the best to use on the trailer, for longevity and protection from the sun’s UV rays? – Jim Sholtis
Chris Marshall: Since the spar urethane didn’t keep water from penetrating the plywood veneer, you could turn to a boat-building finish such as epoxy and give that a try. Honestly though, Jim, I would be reluctant to use (what I’m going to assume is interior-grade) birch plywood on what is essentially an outdoor project. Interior-grade plywoods aren’t manufactured with the same types of water-resistant adhesives as exterior-grade sheet goods. And, even more, a project that will have water driven against the finish every time you pull the trailer when it rains just exacerbates the chances that the adhesive or wood finish might fail. Marine-grade plywood or even a exterior-friendly lumber (such as redwood, cypress, cedar, teak, mahogany, etc.) would probably have been a better material choice here. Going forward, I would probably avoid rainy days when pulling that good-looking trailer around.