Everyone who owns a fibreglass tank has experience (or soon will) with sealants! Ask ten different people, get ten different answers. Kinda like Amal vs Mikuni, points vs electronic, etc...
I have two glass Roadster tanks. I had the black one professionally lined by Ken Armann in Cali. Didn't want to mess with all the toxic chemicals myself. I had it done two years ago and it's been fine.
I also have an original Fireflake Royal Blue glass tank that is currently unlined. I was thinking of using Caswell Plating's two part epoxy liner. I've heard good things about that product.
I bought a bass-boat-red fiberglass tank on Ebay for next to nothing for a spare. I lined the inside with the Bill Hirsch stuff but have yet to put any gasoline in the tank. Therefore, I can't give you an accurate account of how well the coating holds up in service.
There are some noticeable differences, however, between Kreem and Hirsch. The Kreem product seems more rubbery than Hirsch. The Hirsch dries to a smooth, shiny finish that is supposed to adhere better to fiberglass than Kreem.
We used the Caswell epoxy on a '68 Fastback tank and it has been perfect for the last two years. It generates a little heat during cure, so don't be alarmed, but it dries to a nice gasoline resistant finish and adheres well to fibreglass.
The epoxy is a better product than the Hirsh liner. But they will all fail if you don't get every part of the tank cleaned and coated. This is the most important and the most difficult. You can't see every surface inside the tank and you must judge the whole job by what you can see. After two years with a repaired tank fuel found it's way through the liner and did more damage than any leak could have. The Hirsh linner is still good and is loose inside the tank witch is a mess . But a pin hole in the epoxy would have had the same results. norbsa