RodneyBD wrote:I would appreciate any advice on repair techniques?
I don't have any first-hand experience with repair of osmotic blisters. I will offer some general advice.
The blister was formed because water was able to pass through the fiberglass resin used to laminate the hull structure.
The first step in repair of a blister is thus to open the blister and allow the water or whatever liquid there is in the blister to drain off, and allow the laminate to become completely dry. The presence of some uncured laminating resin might also occur in the blister. That resin has to either cure or be removed.
In the illustrations above many of the blisters appear to already have been opened.
If the depth of the blister was limited to only the upper layer of the fiberglass laminate, that is, the gel coat resin layer, then the strength of the hull and its integrity is not really affected. The depression created in the gel coat layer can just be filled.
If the size and number of blisters are small, you could attempt to repair each blister individually using a color-matched gel coat resin with special additives that will permit the gel coat resin to cure to a hard layer in the presence of air. This resin type is called a gel coat paste, and it usually contains some other ingredients (such as a wax) so it will form its own top layer to block air out, letting the polyester resin cure to full hardness.
If you were skilled in this method the repair would become cosmetically invisible. Also, the location of the blisters on the transom and hull side will make getting them nicely filled a bit difficult due to the effects of gravity. You will have to work out some process to prevent the repair resin from dripping out of the depression.
If the size and number of blisters is large, their repair individually with color-matched gel coat resin paste will become tedious. At this point you have to decide if the cosmetic result of the repair is extremely important.
If the cosmetic appearance is not very important, you could fill the depressions with some tinted epoxy resin with a filler, but eventually the color of the epoxy repair will shift due to exposure to ultraviolet radiation from the Sun. The repair will look very splotchy. Filling the blister depressions with an epoxy resin fairing compound can also be done.
The general remedy to cover the non-matching color of the repairs is to apply an entirely new top coating over the general area of the repair.
The top coating could be a new application of gel coat resin, but I would expect that would likely be beyond the realm of a do-it-yourself project.
The top coating could be an application of a marine paint, perhaps a modern (and expensive) marine coating like AWLGRIP paint, which, again, may be beyond the do-it-yourself level of skill.
A less expensive marine coating like TotalBoat Wet Edge Topside Paint, which is more likely to be possible to accomplish without special sprayers and high skill in the application, could also be used.
Any method of filling the blister holes will require filling, fairing, sanding, and the top coating with a water-impermeable layer to stop further occurrence of water infiltration into the fiberglass laminate hull.
