Blisters. My daysailer is stored on a trailer. The hull is lexan plastic, but the rudder and centerboard are old school polyester resin and glass matt.A final photo, close up of the bumps, again these are smaller than a pencil eraser.
Interlux Watertite is much like the product you described. Easy 1 to 1 mix. One part is white the other dark blue so you know its fully mixed when you have an even baby blue. Almost peanut butter consistency, but lighter and easier to mix than that, won't sag even over-head. $60 USD here at the SBO store but you can fill hundreds of blister repairs, especially with shallow blisters. Pot life is good, you'd never mix up the whole liter at once... a few tablespoons on a plastic board with a plastic putty knife and you'll fill several square feet of ground out blisters in plenty of time before it kicks. Sands well too.When we did Vigah we used a special epoxy resin by Magnolia. Two part and it did not sag. Bloody awful expensive though. It is still available and used in aerospace. Was really easy to work with as it was a two part paste with a descent pot life even in the Florida heat.
@rajhnsn , you are spot on with the majority of what you said, but I have never heard that uncured resin is part of the equation. I am not saying you are wrong by any means, I just haven't read that from any source. It is a likely explanation if PE resin wasn't fully and thoroughly mixed with catalyst. My understanding is that PE resin is not totally waterproof at a microscopic level, it allows water molecules to work into the laminate and this is more typical in 'loose' layups where chopper guns were used, higher content of air bubbles, and poor quality resins that were used during the fuel crisis in the late 70's that also persisted into the 80's. I dunno... but what you said seems to make sense, what else would cause the chemical reaction with the water when the resin (if it were properly cured) should basically be inert plastic? Food for thought.It is my understanding that blisters are caused by resin that did not activate in the lay up process. The ones that are on the surface are usually starting in the mat layer of the fiberglass. Minerals and other chemicals in the water that are absorbed through the gel coat and react with the under activated resin giving off a gas that eventually causes a blister. Depending on the number of blisters, the best process, in my opinion, is to have the gel coat removed, sand all of the areas of unreacted resin (they show as white) and re-coat the bottom with vinyl ester resin (several coats, I did 11), then possibly use a barrier coat, then bottom paint.
If you do not take off the gel coat, how do you know where the areas of potential blisters are? If you only treat the ones you see, then you have the potential to have more of them pop every year.
For a yard to do this job it will cost between $10,000 and $15,000. I did the job myself for about $2,500 which included gel coat peel, tool rental, resin and advisement from a fiberglass expert in the Rock Hall, MD area. We did the job more than 8 years ago and there are no new blisters since. It is a job you will only do once in a life time not because the blisters may never return which is also true but because the work is brutal. The price is right however if you are up to the job.
https://www.westsystem.com/instruction-2/epoxy-basics/clean-up-removing/I have never heard that uncured resin is part of the equation
@BobbyFunn , good points but that is epoxy resin/hardener... we were talking about un-cured polyester resin in the laminate causing osmosis.https://www.westsystem.com/instruction-2/epoxy-basics/clean-up-removing/
DO NOT dispose of epoxy hardener in trash containing sawdust or other fine cellulose materials-spontaneous combustion can occur. Clean epoxy resin or mixed epoxy residue with lacquer thinner, acetone or alcohol. Follow all safety warnings on solvent containers. Clean epoxy hardener residue with warm soapy water.
3m safety sheet for bondo hardener says to wash with water with contact.
If the batch isnt mixed thoroughly the hardener in the resin will be diluted with any water it comes into contact with, could be osmosis, or maybe even condensation on the inside.
So you have a hard plastic substrate with pockets of water soluble liquid mixed it. The problem could be the layup or the gelcoat.