i had a 70's dunstall glass tank. I lined it with caswell's kit. It weighed 2 pounds more afterwards. The caswell kit is too stiff and brittle. It got one little crack in it and gas got behind the sealant and started disolving my glass tank.
First clue, my carbs started getting stiff and sticking. Cleaned them and then a couple weeks later they did it again. Then i hit a large bump and the tank started leaking.
I patched it with some glass and resin but that didn't last long. I ended up having to replace the tank. I had the replacement made using an epoxy instead of resin. It was vacuum bagged too.
Using an epoxy was said to fix the issue that modern gas has with glass tanks. I'm able to keep fuel with e10 or whatever that corn based #$%* is, in the tank year round. The tank hasn't flinched since.
so,
- don't use caswell sealer.
Fiberglass tanks can be built to hold modern fuel. Just not airtech's.