Sure it's not so good to use bog at all and purist and skilled panel beaters can make bent panels flat. For the less skilled many of us resort to a bit of bog (polyester resin).
After beating out a hail damaged car the best I could I used bog. I'v really come to respect the skills of panel beaters. To my dismay after about 8 months the stuff has shrunk back. In most cases the fill depth was at max 2mm and most less than 1mm.
Are there some brands of bog which are better than others?