Posted for "Denny" <d.duello@www.mebbs.com>:

Hal,

To remove vapors from your tank:  1.  Select a flexible hose with an O.D.
about a half inch less than the I.D. of the opening in the top of the tank.
2.  Taking advantage of any existing bend in the hose, extend one end of the
hose to the lowermost point in your tank, and secure it in that position.
3.  Connect the free end of the hose to your shop vac and turn it on.  The
concentration of the vapors in the tank will drop immediately. Let the vac
run for hours (or days) until you build confidence.  It's necessary to allow
fresh air to enter the tank during this operation (thus the half inch
difference) while maintaining a reduced pressure inside the tank.  Vapors
which might have seeped beyond the tank will be drawn back out via the same
route.  Decrease the half inch dimension if you want to further reduce the
pressure inside the tank (the air flow will increase) during the operation.
Converse, converse.

Denny