I need to make sure it won't explode and was thinking about letting it
sit in the sun open for a few days then fill it completely with water.

Any other ideas?
I've used the exhaust port from a shop vacuum piped into the tank and
just let the vacuum run until you're done welding. Moves enough air
that there is no build-up of fumes..... I've also used water on
smaller lawn mower type tanks.
Ken.