I wanted to share my experience with Evapo-rust, I recently had my tank painted and upon getting it back i discovered that while prepping it for paint they had washed the tank with water. The entire inside was covered in a coat of slight rust. I went to the auto parts store and looked for the chemical, it was 10 dollars a quart! I bought it from harbor freight for 100 dollars for a 5 gallon bucket. After filling the tank 2/3rd's with evapo-rust, I placed a test plug from the plumbing supply in the filler neck. I rotated the tank for about 10 days (I know- overkill) and after draining I washed it out with a few bottles of dry gas. Result- No rust! and it looks like new, great stuff and thanks for telling me about it.