If you are using a plastic filler in that area you could try Metal To Metal filler from Evercoat or just lead fill it instead. Both would be better for areas that are stressed. If it is just primed and painted, someone else might have a good answer for you because I am no body work expert. I would think the goal would be to make what ever was used there more flexiable. Might be as simple as adding a flex agent to the mix.
Do you have pictures of the exact area you are talking about?
