I prepped the tank first by shaking dry wall screws around in it until I thought I’d collapse. I detected very little surface roughening from this, but a little. I just couldn’t do it anymore. Then I dumped a quart of white vinegar in it and sloshed it around thoroughly. I left it in for a few days, sloshing it and turning it to sit on a different spot many times. Then I used a quart of paint prep, same technique. I drained the tank and left it for over a week. It felt completely dry. I could detect no residue at all. It smelled clean, faintly of vinegar. Now, a week after coating the tank, I checked it to find the coating had “ “pimpled “. This is usually a result of surface residue when it happens to paint. I plan on using it anyway, hoping for the best, expecting the worst. Disappointing.