there are several tank sealer products on the market. for them to work you must prep it properly.

clean it first with a detergent mix and some small sharp gravel as a abrasive and shake the tank well to knock all the loose rust and crud off.
use a rust converter to treat the rust follow the directions.
then apply the tank sealer
the POR15 product is a 3 step system and all products in the kit need to be used or it wont work properly.

r simply make a new tank.
if you get a old one i recommend you treat it with a tank sealer product to get a never touch it again repair because the fuel we have today is just crap and readily absorbs moisture which creates the problem.

I have a good used one coming from D2Gary. Thank you all! Elton