Radiator shops boil in a soda solution. They used to use lye, not sure what they have now. We usually take the tank to get boiled, then we clean it good and use the POR-15 tank sealer. On cylinder tanks we set them on rollers and use small granite for an abrasive and tumble to clean the inside.
POR-15 says that if you follow their process, you only have to clean the tank out with their cleaner, removing the rust with other methods is not necessary. I've never done one that way, but I have no reason to doubt them.
