Air bubbles?


WCHA member #8947
Just a quick question - I apologize in advance for my ignorance. I have a 17' Redwood strip solo boat, that unfortunately, has to be kept outside (I rent an apartment on a lake with no garage, so in order to have it at "home" and use it almost every day, I have to keep it outside. It is in the shade, and covered most of the time). Just today I noticed what looks like pockets of air between the wood and the varnish, running with the grain on the exterior underside of the boat. My current hypothesis is: the other day I left it right-side-up, and got up in the morning to find it had rained a small amount, and the water had collected inside in the bottom. I drained it, and didn't take it out that day. I'm thinking possibly, some of that water filtered through the strips from the inside, and maybe got between the wood and the varnish, and vaporized/expanded in the sun (I had it out today) or something??? Granted this is coming from a person who has some experience canoeing, but no mechanical knowledge whatsoever. I've attached a picture showing what these "bubbles" look like. Basically, I just want to know what I should do to A.) prevent further bubble episodes (I know, I know, keep it inside, but I just can't stand to think of it sitting in a garage hours from here while I look out my window at the lake every day. Sigh...) and B.) what maintanence steps should I undertake to protect the wood, and keep my boat in good shape?

Sorry for the long winded post. Thanks for your expertise. I just want to be a responsible wooden boat owner.



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Most strippers have layers of fiberglass & resin or epoxy on both sides of the wood. They are then varnished with marine spar to provide UV protection and a nice glossy surface.

The pictures were difficult to diagnose. If the water got between the varnish and the epoxy layer, you would be able to sand out the bubble (without going through the fiberglass cloth) and re-varnish.

You can see some of the weave of the cloth in your photo. It's possible that the epoxy/resin layer was compromised allowing water into the wood. If the water got beneath the fiberglass, it's a whole different story. Over time, you might see some staining of the wood. Check your boat carefully for cracks, wear spots etc. then re-coat.

Another possibility (though I don't think so from the photo) is that the boat was covered with a polyurthane varnish rather than traditional spar. That has a tendency to bubble in the sun. The long, thin shape of your bubbles leads me to believe that this is not the case here.

For now, you might want to let it sit for a day or two to see if the spots do anything. If it has been as hot there as it has been locally it may just be moisture in the wood trying to escape.
Unless the bubbles are full of water (which I doubt) I don't think water has anything to do with it. It's unlikely that water would get into the core of a well made stripper in such a short period of time, even more unlikely that it would migrate through the strips and produce damage on the outside and very unlikely that it would get in, work it's way through and not do some really serious damage (like fracture or delaminate the outside glass. Chances of it working it's way cleanly through both the inside and outside layers of epoxy/fiberglass and the wood core, yet doing it's damage to the final layer of varnish on the outside are so small that they aren't even worth mentioning.

There is some evidence of what looks like it could be heat damage. A good example is that patch of small whitish dots about halfway down the right edge of the photo. They show the weave of the fabric and are most likely small internal fractures in the fiberglass layers. A dark hull like that can really heat up in the sun if left on a cartop or upside down in the sun. The wood expands if it gets really hot and the fiberglass can't expand enough to keep up with it and you can get patchy areas where the laminate has been stretched a bit. They generally aren't a big deal from a structural point of view, but are just about impossible to get rid of.

Varnish, as long as it's not decades old, will stretch a little bit, but it may or may not come back to it's original size and shape once the hull cools off and contracts. It's hard to tell without seeing it in the flesh, but I suspect the hull expanded from heat and during the expansion/contraction process the varnish got stretched and has delaminated from the epoxy surface, forming those longish bubbles.

If that's the case, you will eventually want to sand the varnish down smooth and re-varnish it. Every couple years a stripper that lives outside needs this anyway, to renew the U.V. barrier that's being provided by the varnish and protecting the epoxy under it. In the mean time, avoiding having a boat full of water is obviously always a good idea for a number of reasons, but I think a light-colored boat cover that blocks U.V. and reduces the temperature of the hull's outside surface may be the best protection.