A slapper can be used to with a dolly behind and it does level the area. It also puts some cross hatching in the surface.
The shrinking disk made the area very flat however the problem I had was that I had to be careful that I didn't shrink the whole panel too much and loose the shape of the panel.
The father in law was a bit sceptical but he did use it in the end, and as Absinth said he used to us the torch and damp cloth. It was also the way they used to remove hail dents.
I must say I was amazed at the result.