I used the POR15 on an irreplaceable motorcycle tank recently, and while I haven't put gas in it yet, the cleaning and sealing process went pretty well. The phosphoric acid etch made the metal inside look brand new, without the need for any ball bearings or rocks sloshed inside. You can leave it that way, as long as you fog the metal with some light oil to keep it from rusting, otherwise it will rust almost instantly. The POR sealer seems to be some tough stuff, and I ended up lining my bike tank with it. My bike sees low miles and gets to sleep in a heated garage, so it will probably last a long time, but I would use the sealer on just about any tank. Just make sure the inside gets cleaned and coated thoroughly or the coating will fail quickly.
