Propane tank PSI

It is not recommended to use ropane tanks for air. According to what I have heard, compressed air usually has a good amount of water in it, and propane tanks are not sealed inside. Again, according to what I have been tolde, continued use of air will cause moisture inside and rust, leading to failure, which could be very bad, depending on the instance. So not my opinion, just what I have been told, but I wouldn't do it.