I'm sure everyone has at least heard that you're supposed to drain your tanks on your compressors, I think I found out why last night.
I was going to do some painting last night when I discovered my compressor did not work. I did some simple investigating that turned into an hour and a half long compressor "autopsy". Turns out a piece of the motor broke off and caused some pretty significant damage to other (apparently vital) parts of the motor. Obviously it doesn't work anymore. It was really cool, however, to see what makes that thing hum.
So, during the methodical destruction, I kept moving the compressor and I noticed a swishing sound coming from the tank. I knew that the tank was supposed to be drained regularly, but I had never been able to do this; the small drain at the bottom of the tank never seemed to work. After I removed the entire drain assembly, I found out why it never worked: there was so much rusty, silt-filled liquid in the tank that it had completely blocked the drain. I don't know how long it had been this way, but the water looked pretty old. The compressor was 4 years old, and it has been in the garage for at least 2 of those years, never able to be drained.
After I removed the drain, I actually had to push through a layer of muck with a screwdriver, at which point the liquid started pouring out. It was a rust colored water, thicker than plain water because of the silt (or something) mixed in, with a metallic odor to it. I was able to get most of it out, and I'd say about 3-4 ounces total that was removed.
So this ordeal got me thinking that this is a pretty good example of why to drain your tanks. I don't know how much, if any, damage there was to the tank, but all the rust could not have been good for it. I have a friend with a cutting torch, and I might get him to cut the tank open so I can take a look. If I do that, I'll post pictures. It was a pretty neat experience, except for having to spend money on a new compressor.