Hey New Guy;
Odds are if you got alot of crud out of there it needs a good cleaning. If you have a local Radiator repair shop they can usually drop it in a tank of cleaner that will clean it and get rid of the rust (and check it for leaks as well).
Once thats done there are alot of good tank liners out there, Eastwood, POR, etc... Odds are that your local auto parts store has a good lining material that you can pour in and swish around.
I've also heard that KBS makes a good kit that includes a cleaner, de-rusting agent, and a sealer. Depends upon how much you want to do yourself. Googling, I found this with a video of the process
http://www.kbs-coatings.com/auto-fuel-tank-sealer-kit.html Personally I like Kano's EXRust for neutralizing the rust, but thats just me. Good luck - here's hoping there are no leaks!
