To really know if you need a barrier coat requires you to know something about finerglass manufacturing. Starting from the outside in, a hull
will have some number of woven glass layers applied to thehull. So long as these layers are fully wet out then extra resin rolled out they are pretty good at resisting water
After the woven layers, in fiberglass there is then almost always a layer of either CSM (chopped strand mat), or mat. This layer is to prevent bleed through of the woven pattern and reduce fairing costs and problems. The CSM is applied by a gun that in one nosel is shooting out resin, and in the other is cutting fiberglass threads to pieces. These are then mixed as they are applied to the hull. This results in lots of little threads oriented every which way, including sticking out away from the hull. Mat is a little better but also has the random orientation.
Finally there will be a layer of gell coat covering everything.
Now that we have the layers in mind, let's work our way back to the inside following the water.
Gell coat is water permeable, it isn't even supposed to stop long term water intrusion. Right below this is that layer of CSM or mat... Remember that random thread orientation, they also act to help wick water into areas of poor wet out (extra resin). Basically the random thread orientation acts like a path for water to follow. Since it is very hard to ensure perfect wet out, there are always pockets that are problematic. The CSM is much worse for this since the threads also work directionally into the hull, not just longitudinally.
Then we have the woven layers. Rarely if ever do you see blisters
here. It can happen, but these are the big, narley structual problem blisters
that can delaminate and destroy a boat. Luckily they are very rare, since it is easier to wet out these areas. The only cases I have seen were caused by improper mixing of the resin in the first place.
What a barrier coat does is place a shield around the entire hull out of epoxy. Since epoxy is highly water impermeable, the entire hull becomes water tight on a microscopic level. Of course this requires a certain thickness to ensure complete coverage (typically about 7 mills, but check with your manufacturer).
There is also the question of what goo the builders are using to start with. For instance most carbon fiber boats are built with epoxy, and thus are naturally waterproof. So they don't need the barrier coat as much.
Vinylester resins are not quite as good as epoxy at resisting water absorption but are much cheaper. They are also stronger than polyesters. Still require a barrier coat, but not quite as desperately as...
Polyester resins are the cheapest out there. They are also the weakest, and absorb the most water. In fact the gel coat we talked about earlier that is water permeable is just a specialized polyester resin. In fact polyester is pretty terrible stuff, but it is cheap
, so builders trying to hit a price
point still use it. Otherwise it has nothing to recommend it. Epoxy barrier coats are required in all applications unless the boat is out of the water all the time.
note that this is targeted at modern boats. Some of the older hulls were nothing but CSM because they really didn't know the downsides to it. This did lead to some major problems with delaminating blisters, and made boats much heavier than necessary. In part because CSM is such a bad structual material.
Originally Posted by zeehag
any boat built after the change in procedure for building fiberglass boats is subject to the possibility of osmotic gelcoat blistering--seems spraying gelcoat makes gaps.... the old way was to hand paint 1/8-1/4 inch of the stuff into the mold
before placing the roving and resin is the most protective kind of gelcoat possible.
the builders are not allowed to do that anymore-not since 1973, when the laws were changed -- must be sprayed now thank your legislators for imperfections, as they are who forced the industry to change from effective to ineffective gelcoat application.
This really isn't true. The application process of gel coat has nothing to do with blisters. Gel coat by its nature is water permeable, so, within reason, thicker gel coat won't really help.
Thick gell coat also has a lot of problems. It is very heavy, and very brittle. Most stress cracks in fiberglass laminates are directly caused by too thick an application of gell coat. I have never seen 1/4" thick gell coat and would be amazed at any builder
who ever had this as a layup