I'm going to paint my car and I know my compressor puts out some water.  So I did a little research and decided to make an aftercooler to help with this.  The idea is that if you decrease the temperature of the compressed air enough, the water will condense and drop out, enabling you to remove it before it gets to your paint gun (or air tools).  Some people use transmission coolers or A/C condensers, but I was concerned about friction loss and I didn't happen to have one of those handy, so I did something different:

Each one of the long runs is 24", so the air travels thru about 14' of copper before it gets back to the tank, which allows for a lot of heat dissipation even without fins.  The line coming out of my compressor is 3/8", so I stepped it up to 1/2" to reduce the friction losses and provide a little more area for heat dissipation.  It goes back to 3/8" just before the water separator, which is one of those cheap ones from HF and automatically drains when the pressure falls off.  All the joints are soldered, with the excepting of the compression fittings at the separator, the compressor, and the tank.  I used 3/16" rivets and 1/2" conduit clips to attach the whole mes to the side of the compressor belt guard.  Here's a shot from above:

You can see that previously there was just a short drop to the tank.  So how well does it work?  I used an infrared thermometer (one of my favorite toys) to measure the temperature on the line coming out of the compressor after about 10 minutes of running and got 285 F.  At the end of the line where it enters the tank, I got 98 F.  That's enough of a drop in temperature to condense just about all of the water, which ended up in the separator.  When I drained the tank, no water came out, where previously I would get a puddle.