Acetone has worked for me. I'm a fan of both Redkote and POR15, but if you use Redkote there are fewer steps and you can dilute it with acetone (or not worry about getting all of the acetone out as you use the Redkote). Acetone may take a while to get through the Kreem, but shaking it around with something in the tank (a chain, screws and bolts, etc.) will hasten the process.