Large flat areas such as roofs, hoods, trunks, etc., should be block sanded for best results. If you do not have a sanding block, go to your nearest hobby supply, buy a block of balsa wood that is at least 1 X 1 but no greater than 2 X 2 inches square on its end and about 3 inches long. If not available in small pieces, buy a larger piece and cut it into 3 inch lengths. Wrap the polishing cloth around the block as you sand. Be very careful using this method around raised areas or ridges on your painted model.
Many fillers continue to cure and shrink over long periods of time.
This is especially true or most fillers used by hobbyists. Whenever possible, fill gaps and low spots with pieces of sheet styrene and cut them as close as possible to the edges of the area being filled. Polyester glazing compound is available at most auto parts stores and, in my opinion, works better than Bondo. Lay the plastic rod along the seam and, using the applicator, glue this strip into the gap. Apply the glue liberally.