I assume you mean body filler (not Bondo). Bondo is good for plastic like bumper fillers, but shouldn't be used on metal because it is NOT waterproof.
ALSO, body filler is not to fill holes or dents deeper than 1/16th in. thick...It is best to think of filler as a "thick primer".
If the dent is no deeper than 1/16th, first sand the center of the dent, as well as an area about 2-3 in. outside the dent with 80 grit Wet-or-Dry. Then get a good quality, waterproof filler (Go to your local auto paint supply store, they will guide you to the right products) and fill the center of the dent, feathering 2 inches(at least) past the outer edge. 2 applications may be required (sanding [180 grit]between coats) because filler shrinks as it cures, leaving a low spot in the center...Allow time to cure between coats! Use a good quality automotive primer after sanding[180 grit] till satisfied dent is hidden. Next you must use an acrylic glaze or apot putty to fill in the pinholes...There WILL be pinholes, do not miss this step. Duplicolor makes a product in an aerosol can called Filler Primer. It can be baught at Wall-Mart. This suff is pretty cool for small imperfections or even to make up for a little amount of surface irregularities, but has no rust preventing capabilities whatsoever. It also has to be left to dry 24 hrs. and wet sanded between double coats for best results.
Remember this: every application of filler, primer, glaze, must be sande or the following layer will not stick..Do not go higher than 220 Wet-or-Dry till ready for paint.
At this stage your ready for paint...How you tackle that is up to you.
If your dent is deeper than 1/16 in., iether pop it to 1/16 from behind and begin, or drill a small hole (for a dent puller) to get it to 1/16, but the hole must be welded closed...NOT FILLED!
Hope this helps. Good luck!
