If it were me I'd just empty the tank and drop a small piece of pull chain in it. Then shake the heck out of it. Rinse well with kerosene or drizzle fuel.
Most parts stores have sealant for motorcycle gas tanks that will work to seal it. POR15 is expensive stuff and if you open the can you have to use it all (I think). I know its $100 a gallon because I used it on a jeep frame once.
I figured the website would be easy for most guys kids to find
I honestly "think" the product works as described on the site and on many other forums but, I just had an itty bitty thing to get the rust off of and wanted to try a product that could be re-used. It worked for my application but it wasn't even as big as a lantern bowl.