When I restored my RCA CT-100, I applied a product called
Vacseal to the 15GP22 perimeter joint, thinking that it might help and it couldn't hurt. I believe John and Bob G. may also have experimented with applying Vacseal and curing in a heat box around that time.
They have done a lot of investigation since that time, and I believe the current thinking is, as Tom says, that you need something like glass or metal. The only proven method is frit glass. The RACS company in France did a frit glass seal on a 15GP22 some time ago, and the last I heard, it was still holding up at the ETF. My 15GP22s are still holding up, too, but that proves nothing, since they didn't leak before I applied the Vacseal.
Phil Nelson