Glenn Ashmore wrote:

"Empty" is a lot more dangerous that full. Gasoline vapors can remain in a
good enough concentration to explode for a long time. The sure way to purge
the tank is to fill it with water to overflowing and then drain it. Any
remaining traces of gas will float on top and overflow first. If you can't
purge and drain it completely that way open the inspection port and drop in
some absorbent pads for a couple of days. Retrieve the pads and blow air
over it for a couple of days to vent the remaining vapors. (use a spark
proof fan.)


How about throwing in some dry ice to displace the vapors?