Sink hole help!

OK guys!
I'm welding in the strip in the middle of my sons Nash after the chop and I have two areas that have sunk in.They are about a 12 in. dia. area on either side of the strip.
My question is this!
I know that by applying heat I can "pop" the low spot out but just where and how do I do that? Do I heat the center and cool the outer ?Or heat the outer and cool the inner? OR what????
Any ideas will help from those of you who have done this before!

No help!

Gussett,
Sorry I am no help on this one! I always get it backwards!!!

BoB

I know when my brothers car got hail damage to it , the body man who fixed it put small pcs. of dry ice in each dent , it popped all the dents out and didn`t crack the paint , don`t know if that helps you or not.......24