Advise on adding a sump to stock tank

When I welded mine I placed a steam line in it for the day, no kaboom.
Then I worked in a Harley shop they use to wash out the gas tank with water put it on the floor put there foot on it place the touch at the opening and it would make a whoush with a little flame coming out and then good to weld on. ( I **** my pants the first time I saw this)
It's a long winter, don't get in a hurry keep it open to the air for a few weeks.

Kenny