Survey says: (the web)
"Is there specific high spots to get out or just and overall area, you mentioned oilcanning so i assume you have a high crown? To get that out i'd use a torch over a biggish area, keep the metal cool enough so it dosn't get to 'blue', then quench with wet rag. Do this a few times and it should tighten right up.
When ever you hear a 'hsss' when apply the damp rag then you're shrinking, it is easy to overshrink then you'll have a problem having to both stretch and shrink, keep the metal from going 'blue' because it is more focused on that smaller area.
To get rid of a high spot then you'd heat the top of the high spot, about the size of a dime, until it is blue and then quench.
To get rid of a silverdollar sized highspot, heat to reddish and work it with hammer and dolly twards the middle gathering the excess metal and then beat the hell out of it with a hammer - well, maybe use some finesse."
I would try to practice this on something for a while first.
