I assume we are talking about a air compressor here? If so, pretty hard to clean out. Are you using a filter/dryer/regulator of some sort? If not that would help. I cleaned the rust out of an old tank (fluid tank not air tank) by putting muriatic acid and gravel in it and then spinning it on a cement mixer for an hour or so. But if you do that to a air compressor tank the rust is just going to come back. Better to use a filter, dryer of some sort.
