What truckernix said, remove the bondo without removing metal. That area maybe weaker do to the previous bodywork, so easy does it. Just make sure the metal surface is clean before you spread any body filler. Body filler that won't feather edge is a sign of poor prep, and or "bad" filler. After sanding out the body filler, then prep for primer. This is where I suggest you "read" the product labels and follow the instructions. With paint jobs costing in the $thousands you don't want to take any short cuts. The paint supplier will explain thier products, and in most cases try to over sell, so do your homework. Last but not least, depending on the size of the area your working, the roof can be kind of tricky, don't stretch or warp the metal. If you don't have bodywork experience, seek advise from a professional, most of them like to share thier wisdom.
