To remove the liner i used MEK or Acitone that you get at your local hardware store.
I have found that it would of been much easier to just not worry about getting the old liner out as the new liner would of attached it self to it. I used KREEM as a kit and stage two is, you guessed it, MEK. What happens is, it desolves the liner and it all goes to the bottom of the tank. Can't get it all out, plus you have a messed up tank on the outside. So, get your tank liner kit, soak it for rust (stage 1) go onto stage two and let the MEK thin out the loose stuff and don't worry about the rest. Go onto stage three. If you like your paint on the tank, protect it the best you can. This stuff can really mess up a pretty tank.