hey guys!! As it is stated avove, bondo is not recomended at all... Poliester is better but it is only recomended for superficial work... NOT FOR FILL dents or anything... My personal rule is any kinda poliester fill cant be more that 2 milimeters deep... (arround 0,1 inches).. and never be applied on a "soft" sheet of metal (sometimes if the skin has suffered a lot, it just moves too much and break any fill you apply to it...
I do use a made in spain poliester & metallic fill traded Roberlo...Not cheap at all but as it is metallic fill, it is much more stable under different temps (the big problem is when at different temperatures, metal expands more or less than the filler causing cracks by movement)... Besides, it suppose to have magnetic propperties that makes the attachment on the metal sheet stronger.
You guys should have a lot more variants for this or every body work but the important thing is to think filler as not a magic tool with you can sculpe the acual shape of the car!... It is always better to do the metal sheet better than to fill more than recomended with bondo... at long, it will be much more expensive
Hope it helps!!