To seal a firewall you can use:

polyester resin
epoxy finishing resin
slow-cure epoxy glue
polyurethane varnish (may need a few coats)
clear butyrate dope (or colored to match the model.
epoxy-based paint (whatever color you like)

All of these are moderately fuel-proof. Keep the fuel exposure low, or keep it clean, and use fuel with 15% nitro or less, and the finish will keep its sealing properties.