If you tank is very heavily rusted, the bolts idea works great. Another method of rust removal is using molasses and water. 2 Parts molasses to 7 parts water. Mix it up well so there is no molasses when you run your hand over the bottom of the container/bucket/big tub and put you rusty parts in there. Smells a bit so put it down the back yard or away from your window close to annoying neighbors is fine. You will get a crust formed or some thick foam type looking stuff on the top after a few days, just remove it. If you leave your part in there for too long, weeks usually, depending on the metal it can eat it away. I've done tanks, mufflers, engine internals etc along with rusty tools etc. When you take it out of the tub, wash it off with water and maybe give it a scrub for the really difficult bits. When you let it dry, it will flash rust because the metal surface will be clean.
I used this method, then followed the por 15 directions on 4 tanks with varying degrees of rust on the inside and the end result was always the same, great.
Now I have found a bubble on the paint on the bottom of my K1 tank. I'm thinking there might be a pin hole starting to develop. This tank hasn't been por 15. Looks like I'll have to wrap it up very carefully so I don't get the 'silver paint' look on my gold tank.