I have also heard and witnessed horror stories about Kream. If I wanted to go that route, I think the Caswell process or the Eastwood process would be a better bet. Better still I think is to strip it back to clean bare metal. I have done them 2 ways so far that have worked very well.
The best way I have found so far involves a clothes dryer, a hand full of 1" drywall screws, maybe 8oz of phosphoric acid (not very dangerous and available form Home Depot pretty cheap), and enough old bedding, pillows, towels, seat foam whatever to fully stuff the inside of the dryer full. This can scrub a near cancerous tank to bare metal.
Remove and plug the petcock. Place the screws and acid in the tank. Remove filler cap and plug the hole. Wrap the tank in old towels or something. Pack the tank and rest of the bedding into the dryer. Keep the tank in the center by packing all around it. Pack it tight enough that the whole chamber is stuffed. Nothing should be able to move around. Turn the dryer on and run it for a long time.
Heat is not necessary and may be a fire hazard (don't know), so disable the heat source in the dryer. Maybe an hour depending on how bad the rust is. Maybe longer. Unpack the dryer. Turn the tank 90 degrees so the screws work a different surface from a different angle. Repack and run the dryer again. Run the cycle maybe 4 times with the tank in different positions. If the tank is real bad, you might want to rinse it out and change the acid and run again. When your done, you need to rinse the tank well with fresh water, drain it, and
immediately fog it or coat the inside with a film of phosphoric acid or some other rust preventative. If you don't coat the surface immediately with a rust inhibitor or put into immediate use with a coating of gasoline the tank will flash rust in minutes and you'll have to do the process again.
The electrolytic process works too. Takes more time and probably more suitable to less rusty tanks. Here's a link to the process I followed
http://thekneeslider.com/how-to-get-rid-of-rust-in-motorcycle-gas-tanks/A third method I have heard gives good results is to fill the tank with feed grade molasses and let the tank sit for a week or more. Molasses is slightly acidic and will eat the rust away over time. I think it is pretty cheap at a local feed store or grain elevator. Might take a week or a month depending on how bad your tank is, but I have heard it works well.
Matt