It's risky business, Will. I used soap and hot water 3 times on a motorcycle tank one time. Then I rinsed it and rigged a tube and a blower to force air through the tank to dry everything thoroughly for several hours. Then, before I did the braze repair, with the filler lid open, I waved my torch past, just to make sure it wasn't going to blow. A 3 foot long blue flame roared out for about 3 or 4 seconds. That did heat the tank enough to clear it, and I did braze it, but if I had the lid closed, I'm sure I would have gone to the hospital one way or another. The only fairly safe way I know of is to fill the tank with water except for a small bubble around where you're working, or fill it with something like carbon dioxide or argon, and be sure no air is left in it.