mr giant gourami;1643413; said:
There are four general types of resins that have been used to seal plywood tanks. I write these in order of increasing cost & increasing "strength". That order is:
Orthophthalic polyester, Isophthalic polyester, Vinylester & Epoxy.
All have been used to waterproof plywood without any sort of topcoat, although I certainly think it would be a good idea when using the cheapest Orthophthalic polyester.
All can be used with or without a embedded matrix of fiberglass cloth. Since the two polyester resins types tend to be the most brittle I think fiberglass cloth is a very good idea with them. It really depends on how much additional structural support one needs.
A fifth choice are the solvent based epoxy paints, which are different from the above. They are thinned epoxy resins with pigments added. They are actually the cheapest to use since they go on the thinnest so you get more coverage. When using these I highly recommend starting with a very smooth plywood since they don't have the thickness to cover irregularities. And it makes the product go farther.
Important point now. Both of the polyester resins and vinyl ester resins will no longer be available in a few years due to stricter VOC regulations. I would be very concerned about using them on a large plywood tank and not being able to make repairs once they are no longer available.
If you find a poly or vinyl ester resin that can be topcoated with epoxy resin then that's a different story.
Epoxy paints will also no longer be available but it is my understanding that they can be topcoated with epoxy resin, which will still be available.
Personally if it were me, I'd use a 100% epoxy resin to seal a plywood tank of the size you are considering. I would put down several layers of epoxy without cloth and then would put cloth in one of the final layers for extra impact resistance against gravel, rocks, etc.
