Ok then I will try my best to explain heat shrinking. In a nut shell if you work with metal and especially panel steel you get to know its properties and limitations...
This knowledge is only gathered through experimentation....years of it.... so to try and explain it here is a bit difficult - so here goes...
The more times a panel is heated and rapidly cooled it will become "work hardened" ...this means just what is says... its hard to work and get to move around the way you want it to.... also it changes the properties of the steel so this means it will become brittle and will crack easier... This is desirable when tool making
( hardening ) but not when working with shaped panels. The first thing to remember is that the panels are formed under pressure... up to 50 tons of it to be exact.. so introducing heat or even sandblating will stress relieve the steel and it will look for the easy way to relax itself... This presents itself as the oil canning that was mentioned ..the easiest way to counter act this is by heat shrinking... Heat shrinking is an aquired art form... Basicially it involves heat a small patch of steel, without moving the acetylene torch at all until a "cherry " or heat spot starts to rise up. This is quickly hammered down after the torch is with drawn in a circular motion working in towards the center or away from the centre of the cherry.. depending on what you are trying to achieve..... The panel is quickly quenched with a damp water soaked rag and even this is applied in a circular motion, mainly to arrest the heat from escaping into the rest of the panel. This action causes the molecules of steel to rush toward the cooling source and this is the main element of the shrinking process. The heated cherry, as its hammered with a suitable dolly backing it, is also prelimarily shrinking the steel.The panel is then cast over with a steel body file and high and low spots are addressed... This is only a brief response to a wide ranging question. I cannot stress enough that this requires years of practise to master perfectly and even after all these years of panel beating ( nearly 20 of them) I am still learning the properties of steel and what happens to it when heat is applied...Cheers streetneat