I have no experience with POR-15 with fuel tanks but I've used it extensively on another project (six wheels rather than two) and found it vexing. It is the world's most difficult finishing/protective product to use and deal with. Unless you have gallons of acetone, be ready to throw away every pot, brush, and spray gun you have after one use. POR "sets" (you wouldn't call it dry because it chemically reacts to the moisture in the air to form a very hard plastic-like coating that seems to be impervious to anything I've ever tried to remove it/ smooth it/ polish it with) in a short time and then it's what you see is what you get. Also, once you open a can, it's almost impossible to get it sealed off again so that the whole can doesn't react and harden -- consider any container opened to have the remaining contents unusable.
When all is well, it seems to bond to metal more completely than any other finish material I've ever seen. The stuff forms what appears to be the perfect chemical and air and water vapor seal. OTOH, I've done big areas and had parts that came out perfect with diamond hard seal coating and other places where it fell off in sheets after 6-8 weeks. I'm guessing it's preparation (isn't it always with paint??) but I'd prepped the entire surface the same way.
POR is sort of like paint for someone who likes to shoot rabbits with a cannon rather than a .22 rifle. I can't imagine trying to work with it as a tank sealer, but -- if by some super process, you could get a smooth, complete, consistent coat and get extra stuff out of the tank before it "went off", it just might make an ultimate liner surface. But woe betideth anyone who has the process go wrong before you finish the sealing job.
And, if it touches any clothing or fabric, it's there permanently. If it touches skin, you have about 3 minutes to get it off with washes of acetone (on your skin - sounds really inviting, right?) and if it's under a fingernail, you look like a punk rocker for 6 weeks.
I'm guessing that when it's right, it's wonderful stuff but IME it's difficult, expensive, quirky stuff to use and when things go wrong, there are no little problems -- only disasters.