Remember, tank liner is not a fix for poor welding. Be sure all of your leaks are fixed BEFORE using tank liner. The purpose of tank liner is to keep the inside of the tank from rusting, nothing more.
First, be sure to get the entire kit, not just the bottle of tank liner
Follow the intructions TO THE LETTER. Do not "cheat" any of the steps. Failure will be the result.
Once you are done turning the tank to assure proper coating of the entire inner surface, be sure to get all of the excess liner out.
I am not sure if this is in the instructions, but use pipe cleaners to keep the vent lines and other small orifices clear. Tape them in and remove them while the liner is still liquid, but after rotating the tank several times. Push a clean pipe cleaner through the vent lines every half hour or so to make sure they don't get plugged up with drying tank liner.
This step is not in the instructions, but it is vital to your success with the newer crappy gas. LET THE LINER DRY FOR A WEEK before putting ANY gas in the tank. Failure to to do this will gaurantee that your liner will fail. This is the number one reason tank liner fails these days.
Removing liner from gas tanks:
This may not work on POR-15 liner, which is an epoxy, but it will work on most of the others. Also, be sure to test your paint if you are trying to do this on a finished tank. Test an area that will not be seen with the tank installed (like underneath, in the tunnel, etc...). Acetone will not harm any catalyzed paint, like urethane, but it will eat right through rattle can, enamel and lacquer (if you are trying to save a custom painted tank from the 70s, be extra careful).
Go to your local pet shop and buy a half-pound of loose large gravel (not the shiny glass beads or rocks, the jagged gravel chunks) or get some small (1/4" or 5/16") nuts.
Go to your local hardware store and buy a gallon of acetone.
GET SAFETY GLASSES OR GOGGLES, WEAR THEM!!
Pour both into your tank, after you plug all the hose outlets, petcock hole, etc. and put on a pair of solvent-proof gloves (Do not put a gas cap on, there may be too much pressure built up from the next step.)
Shake the tank enough to move the gravel around. Cover the filler hole with your hand to keep from splashing yourself in the eye with solvent and debris. Release the pressure when you feel it building up. Keep your face clear. Also, keep a bucket handy if you need to pour off the liquid to check your progress.
You can leave this mixture in the tank overnight or longer, shaking it occasionally to loosen the liner.
You can also turn the tank on a rotisserie of some kind if you can't be there to turn it (make sure ALL holes are plugged in this case).