Phil, my Porter Cable instructions say to drain the tank after each use.
When I'm not using my compressor, I keep the tank's petcock open. I never store compressed air in the tank. With the petcock left open, most moisture that accumulates in the tank drains out via gravity. In use, I run the compressor about a minute with the petcock still open. That tends to force the last traces of accumulated moisture & rust out of the tank. I then close the petcock & use the compressor.