Bob, the POR products I used was "Marine Clean" and "Metal Ready" to clean the inside of the tank. I then used about 3 handfuls of fine blue metal gravel ( from a handy roadside stockpile
) and shook and rotated the tank for about 5 minutes. After shaking out the gravel, I then washed out the tank with water and gave another treatment of "Marine Clean" and "Metal Ready". The inside of the tank is now immaculate! Shining a torch inside reveals steel that looks brand new. POR also have a fuel tank sealer product that will seal pin holes and the like, but I don't require that in this case.