I filled mine with water several times and let it sit in water for about a year.
That was because it took me that long to get around to starting it.
It was open to the air through the fill and vent holes the whole time.
I did let a little water out just before I cut it and yeah I was a little tense but it went smooth.
The next time I do it I will fill it with water a couple of times and then fill it with exhaust gas from a good running engine.
It can be dangerous and use caution. Now mine is a tank that was open to the air for decades before I cut it. I would use more caution on a newer tank but I will do it again.
Be smart and make something others are to timid to attempt!