Leaking Fiberglass Fuel Tanks

I have been looking at different "sloshing" type sealers for the inside
of some old fiberglass tanks installed in the wings of a Cessna.

What is the current thinking for the best sloshing compound for sealing
fiberglass tanks if auto gas is used???

I have seen the epoxy resins from a few sources (including Spruce) as
well as some polyurethane "rubber" coatings.

A couple of points:

Filled these 40-year-old auxiliary fiberglass tanks one time about
2-years ago. They started leaking within a day so I drained them and
have not used them since. I believe they were always used with Avgas
prior to that as I purchased the mogas STC for the airplane.

All hoses were replaced about 2000 and based on location, the leaks
seem to be directly from the tanks. Hoses will replaced again when
tanks are removed.

I always test the mogas I use for alcohol and in the past three years
have never found any but I am concerned I might some day get some gas
with alcohol below the level I can detect.

I will be able to take the old tanks out of the wings to clean, slosh
and test them.

Thanks in advance.