Why not just use the POR-15 Tie Coat primer????
Made for the job, by the makers of POR-15
TIE-COAT PRIMER
Then any product you want to create the fuel tank finish. I used 3M seam sealer in a bag. Only problem is it uses a special gun to apply, but it gives a perfect factory finish. You can then paint it if you want to any colour you fancy.
You can even 2-pack on top if you want…..
Mick