Some old school ideas: If you plan on painting it, then sandblast it inside and out. Some people are scared to do this, but i've done it with great success. Just rinse with lots of water before and after. Another old school plan is to put an old bicycle chain in there and shake it around. This should help loosen up rust that you can then flush out.
I have taken the tank off of a mates bike and flushed it out with pool acid which worked like a charm and got it so clean inside that you could eat off of the thing. then you rinse it out after that with some gas and your off. Just DONT leave the acid in for to long