Originally Posted by Slow & Low
PS. - I purged those tanks very carefully first with air, then with water - I am not promoting this sort of thing - take them to your Fire Department and ask for their help
Liquid propane in it's gaseous state is heaver that air. Fill the tanks with compressed air, invert them (valve down) and open the valve. That should get rid of any propane. You will still have to deal with the stink from the mercaptan (the stuff that makes it stink) though. Flushing with water is ok, just follow up with some alcohol to remove the remaining moisture.