water "spotting" inside Kevalar canoe

I have two kevlar canoes that have develop whitened areas on the inside, under air bags where wetness is sustained for as little as a few hours. One is a Freedom Tripper 17 built with using epoxy resin in 1998. The other is an Apple Line tandem from 1970-80-90 (when Apple Line existed) built with unknown resin. Neither boat appears to have any sort of coating over the interior of the canoe. I am concerned because this is not a superficial spot. The resin appears to have been chemically altered.
Some spar varnish should seal it up but I am concerned about future patches with epoxy resin not sticking to the urethane which will definitely be hiding in the weave of the cloth after sanding.
Anybody out there familiar with the problem? Solutions?
Will epoxy stick to urethane remnants?
Thanks, Pete.

There’s another Pete here who I think knows the answer, and he may chime in after a while. There’s another guy of equal expertise but failing health who has pretty much disappeared from these boards.

One thing I remember is that this topic has come up before, and I seem to recall it’s a rather harmless phenomenon, but I won’t promise I’m remembering correctly. I have a Kevlar canoe with no gelcoat, and it too has substantial areas of whitening on the floor inside, but I’ve never worried about it. Maybe ignorance is bliss, but I see no need to seal a surface that appears to be structurally intact, and if I ever need to make repairs, I think sanding down to fresh material should allow good bonding. When a definitive answer is posted, I’ll be interested in seeing it.

Many Bell canoes were made with a vinylester resin that was notorious for water blushing which was often quite ugly. That would go away by wiping with an acetone rag, but unfortunately would come right back with repeated water exposure, at least in my experience. I applied a coat of marine varnish to the interior floor of several Bell canoes to stop that annoying process. I have no idea if this is the same phenomenon, but it is probably fairly harmless to try a little acetone on a small area to see what happens.

On many composite boats, the weave of the interior layers of cloth is not fully filled with resin. Filling the interior weave is not really necessary so long as the interior fabric is solidly bonded to the underlying layer. Filling the weave with resin adds weight and additional resin expense. But I have had a number of canoes with interior aramid layers in which the resin partially abraded off the raised aramid fibers, and then the aramid would start to take up water due to its hydrophilic nature. This was manifested by the interior aramid becoming visibly damp, and it would stay that way for a while before drying out.

If you feel that this has occurred, or the interior layer of resin appears to be disrupted enough to expose fibers, I would probably apply at least one coat of epoxy after a through cleaning and drying. The good news is that epoxy will pretty much bind to any resin that has been used for composite boat construction including vinylester and polyester resins. Of course, if you want a smooth interior surface, you could apply enough epoxy to completely fill the weave of the interior cloth layer. Covering the epoxy afterward with a coat or two of marine varnish or urethane may not be absolutely necessary but is probably not a bad idea.

I posted the same question a couple of years ago. Charlie Wilson was kind enough to suggest a wipe with acetone. It works great! Not a permanent fix. Not sure that there is a practical one.

As stated above, use acetone and a nylon brush to remove the blushing. For the inside you can use a spar varnish or urethane to preserve it.

I have had very good luck with Systems Three WR-LPU. It bonded very well to my unpainted kevlar interior, preserved the look, and does not develop blushing due to prolonged water exposure. I chose this since it has uv blocking properties, is flexible enough, compared to spar varnish will not break down and become tacky overtime, and has a cross linking hardening additive.

I bought a used 1989 bluewater that had severe blushing, it would only need a splash of water to develop new spots. It also had some uv damaged sections, chalky epoxy with dark weave. The interior weave was completely filled except in those areas, which epoxy covering the weave would flake off. Also when I would go with my dogs, they were noticeably scratching up the interior. After a few coats it looks great, and has put up with my dogs and dragging gear around with no marks. It was not easy to remove all the blushing and chalky epoxy when I prepared it however.