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).
Go to your local hardware store and buy a gallon of CLR (Calcium, Lime and Rust remover).
GET SAFETY GLASSES OR GOGGLES, WEAR THEM!!
Pour both the gravel and the CLR 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 CLR and gravel chips. 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 CLR to check your progress.
You can leave this CLR/gravel mixture in the tank overnight or longer, shaking it occasionally to loosen the rust.
Once all the rust is removed, you can line the tank according to the instructions in the liner kit.
Removing rust from gas tanks - Plan B:
Fill the tank with water.
Take a piece of steel re-bar long enough to protrude out the top of the tank, and tape up the bottom 1" so as to insulate it.
Place a wide-mouth plastic funnel in the filler neck and put the re-bar in through the funnel, so the funnel and the tape insulate the re-bar from the tank.
Get a 10 amp battery charger, preferably one with a current (amp) gauge, and connect the negative clip to the tank and the positive to the re-bar.
Pour salt in through the funnel until the current climbs to 10 amps, and wait approximately 2 minutes.
Check the tank. Continue if necessary, but no longer than absolutely necessary.
Rinse the tank several times, and dry thoroughly. (perhaps some WD-40 to displace the moisture would be helpful.
The idea is that you electroplate the re-bar with the rust from the tank walls.