fixing bondo

Any good Ideas on fixing cracking bondo in areas where its put on heavy? By the rocker panels on my ford the bondo is starting to show thin cracks around the door. From shuting the door maybe? Any suggestions would be helpful.
thanks

 

Take it out and repair whatever is wrong under it. Filler doesn't normally crack if used correctly.

 

Bondo that's been put on right only cracks if the metal underneath is flexing, so you need to rectify that first by grinding out the bondo, repairing whatever is flexing, then redo it.

 

Both replys above are good information-
I'll add this-Check a test area and see if it is white pink or yellow[bondo]or if it is Silver as in the HA-"metal" aluminum based fillers-if so, that is your trouble.
That stuff will not last at all [in any climate] but in the COLD of Minnesota it will be much worse at cold cracking.

 

It looks to be yellow bondo. I dont want to strip it all down because its a very small part at the bottom of the door frame by the rocker. Other than the two small cracks it looks to be done very good.