The tank on my 340U was badly coated with old smelly gas residue, and thick rust granules when I bought it. I first filled it with hot water and detergent to remove any oil coating. I"m not sure how much that accomplished. Next, after draining all of the water from the tank, I put in a gallon of full strength muriatic acid that I bought at an Ace hardware store - same stuff used to clean cement off of brick, etc. I rotated the tank to make sure each side was immersed for at least 10 minutes, two times. When I poured the acid out, it was an extremely dark orange color. I flushed the tank several times with water, and then hooked my shop vac to the tank inlet with duct tape (I opened another hole that the gas gauge was connected into) and blew air thru it for 20 minutes to dry it out. The final inside surface was clean, and shiny metal - the residue and rust had dissolved into the acid. I poured a couple of cups of diesel into the tank to coat the surface and hold off rusting until I got some gasoline in there. I had been given a price by my local radiator repair shop of $120 to do this job. I found in my area, at least, few places clean tanks any more due to safety and environmental concerns. A gallon of muriatic acid cost me less than $10. However, be extremely careful handling it, wear eye and hand protection. It is also safer not to dilute the acid. The same radiator shop told me he doesn"t do, or recommend coating cleaned tanks - it"s apparently too easy for the coating to lift back off the acidized surface if even the slightest edge is left exposed. He apparently had enough problems that he stopped doing it.
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