How can I clean an air compressor tank?
How old is it? Have you been draining it daily (when used), or not?

If elderly and it hasn't been drained regularly, buy a new tank, remove this one, drill holes in it so someone else doesn't try to use it as an air tank, and junk it. If that feels overly drastic, get it hydro-tested. Air tank explosions suck...

If not too old or old and regularly drained, you could either not worry about it too much, or try to flush it out by adding water to the tank. If you have any reasonably large openings (doubtful on a small compressor tank) you could drop in some chain to serve as a "scrubbing" device to help free up loose rust as you turn the tank over and slosh it around. This will require removing the compressor from the tank. If (lacking any large ports) all you are doing is adding water and then blowing it out, you can leave the tank upright and just slosh it back and forth a bit before building pressure on the compressor and opening the drain valve to dump the water (by adding water you will be diluting the "rust slurry" from just being the water that condenses out of the air as it's compressed and the rust in the tank to being something like half a tank of water and the same amount of rust, so you might get more rust out.) More rust will form, as there is always water in the air; in most climates, enough to condense out in the tank, unless there is an air dryer between the compressor and the tank.


If you just don't like the look of the rust in your daily draining - get over it, it's normal. While you might be able to reduce it by somehow coating or treating the inside of the tank it's a fair amount of work and expense for not a lot of benefit.
If you are getting rust in your air, use a filter.