Tank Sealer is formulated and designed to stick to a bare surface that has been prepped with KBS Klean (cleaner) and RustBlast (acid etch). With that in mind, it is always a good idea to strip out the previous sealer with KBS Strip.
Directions for Stripping Out Old Tank Liner
Use KBS Strip for the most effective removal of an old failed coating. Pour some KBS Strip into tank and carefully rotate tank to allow contact with all sides. It may take multiple applications of KBS Strip to finish the job completely. Old Sealer may come loose in big chunks or small pieces so use a long tweezers-type tool to help remove it from the tank. After stripping is complete, rinse tank generously with water and proceed with the KBS 3-Step System for Fuel Tank Preparation & Sealing. (NOTE: When stripping a Fiberglass Tank, surface contact with the stripping agent should be kept to a minimum to avoid the attack & weakening of the fiberglass resin.)
After you have the previous liner out of the tank, you can proceed with the KBS 3-Step Tank Sealing System.
Please check out the Complete Tank Sealer Directions.