So decided to do it the old fashioned way. Some gas in the tank, nuts & bolts in there for about 15 min, mixing it around. Then the acid about 35% about 1/3 gallon, face shield, overalls, gloves etc. 30 min later after swishing around be very careful, looked good. Dumped it into some water with baking soda to neutralize it. Flushed out the tank with water/baking soda then gas. Force dried with the wife's hair drier. Looked inside, it had turned orangey / rusty when it dried, WTF. Thought I had done it correctly, what did I do wrong.
BTW already tried Rusteco, long story $90 left it for a week, don't think I had enough of it, probably needed $180 worth for my tank, it's a 6T tank I've been told, almost 5 gallons... I wouldn't pay $180 for Blue label far less weak acid..