sometimes you get what reffered to as flash rust from a hydro test or just a tiny bit of moisture. My steel tank had a small amount of rust after its first hydro and I cleaned it out myself. If you go and take the visual tank inspection class you can most likely use your steel tank as your test tank and clean it yourself. My LDS lets me clean my tanks there and I just have help him out and do a couple of shop tanks in return. The class pays for itself very fast depending on how many tanks you own.