A key component to cleaning out a gas tank is muriatic acid, which is really hydrochloric acid. I got 2 gallons of this for 10 bucks at our local garden center (they use it for cleaning concrete). The instructions that came with the tank restoration kit said to dilute the muriatic acid 20:1. Since it was already only 30% HCl, that would have made the acid very weak indeed. The instructions that came with the kit made it clear… “DO NOT USE CONCENTRATED ACID!!!” which I interpretated as meaning I’d better use concentrated acid, or it wouldn’t work. I did some tests and, sure enough, I doubt that the concentration recommended by Eastwood would have ever worked for my tank. So, I compromised and cut the acid 50/50, followed by a full strength cleaning for some of the tough spots. At one point, I laid the tank down and forgot to install the drain plug. The acid poured out and ran down on my foot. Some quick work by my wife with a hose, however, saved by my foot and the shoe. I wore rubber gloves and eye protection throughout this process.
A rubber stopper worked well for plugging up the neck to keep the acid in place. After following all the steps (see next page) the gas tank was VIRTUALLY PERFECT! I was really impressed with this restoration process.