Vinegar worked great form me gwolever. My tank was really bad so I let is soak for 3 weeks in a the hot tool room (the hot temperatures speed the process). If your tank is not as bad, then it shouldn't take more than a few days. I filled mine to the top and placed the tank into large plastic bin in case there was any leaks (unlike your type of tank, I had to use duck tape with nylon ties to plug the petcock hole). I filled it to the very top so I wouldn't have to turn the tank over to get full coverage. Then I simply changed the angle front to back every other day so the vineger reaches the top of that part of the tank. I haven't used any type of treatment (coating) since the tank looked to be in good shape and didn't seem to have any leaks. Otherwise, leaks would have been evident during the vinegar treatment. I personally would only use a coating if leaks or metal weakness was an issue.
