I use 3M 847. I would say most any fuel-resistant gasket sealant would work fine. Last time I worked on one I only sealed the screw threads and left the gasket dry. It weeps slightly, so I will seal gasket and all next time.
-Ben
I have not tried it on these tanks, but I would use the No. 2 Form A Gasket sealant, not the No. 1. The #2 is not a hard-setting compound like the #1. I’m sure others will have recommendations as well.
-Ben