I agree with Nick. Evaporust or Metal Rescue.
Take a piece of bike chain about 8" long and put it in your tank and shake it up to remove all the loose chunks. (You can use rocks, or nuts and bolts, but a piece of chain is easy to get out)
Fill the tank with evaporust and let it sit for 24 hours (or more. It won't eat through metal.)
Pour the evaporust back into the jug and save it for later. It'll keep working until, well, until it doesn't anymore.
Then throw away the Por 15. Or return it. If you use it, you might as well throw away the tank. Same goes for Kreem. I've used them both. They absolutely suck and will cause nothing but problems. Google it and you'll see. I don't know how those companies stay in business.
Get Caswell. It's a clear epoxy sealant, it sticks to rust, covers holes and cracks, and NEVER comes off. It's beautiful. It's worth the money, and it just plain works.
I've done at least 20 tanks with it over ten + years and have never once had a tank leak or had the Caswell separate from the tank.