Body Tips by Todd Osborne
reprinted with the authors permission
published 2007
Here are some other tips that will speed the process and give good results (unfortunately I learned some of these after I painted my roadster).
I highly recommend using a 16" sanding block, available from body and paint supply stores. Advantages are long surface (less waves) and lots of sand paper surface (more material removed per stroke). The process I have used with good results is as follows:Rough out body as Paul suggest, use hammer and dolly, go slow, it is easy to stretch the metal. What you are trying to do is push the metal back where is was, examine the damage carefully, try to determine the direction the metal moved when the dent(s) were formed. The hammer or dolly are used to reverse the damage process. It took me a long time to get this to work but low spots can be removed by a dolly from behind and hammer in front (ie, you don't need to hammer lows out). For fairly minor damage, any low spots will be accompanied by high spots, this is because the metal is just distorted, not stretched. The key here is careful examination, you can easily see the lows but there are many highs around the low that are hard to see. The dolly is held firmly behind the low and the hammer is used to *lightly* tap the highs down. It is surprising how easy it is to move sheet metal when the force is applied in the right place. Try to get the sheet metal as true as possible prior to applying the bondo, *no* hammering after bondo is applied this will cause cracks and weaken the bond. I try to get the surface smooth enough so that the bondo is not more than 1/8" thick> (there are places on my roadster that are much thicker than this, but it's not my fault... really!). There is a point of diminishing return with the hammer and dolly, eventually the metal gets work hardened and is difficult to move, so again go slow.
Once the metal is roughed out disk sand area to be filled with 36 grit open coat sandpaper, then clean surface with something like Jasco metal etch (I use a 20:1 Water:Pool acid mixture, wear goggles & gloves) rinse thoroughly with water. Both the rough sanding and acid etch are necessary to get a good filler to metal bond. Some notes about bondo: don't use it to fill holes (these need to be welded shut), don't use it over rust, sand off all rust first or if serious corrosion replace the damaged metal. Quality bondo is good stuff, it is flexible, has nearly the same coeficient of thermal expansion as steel, bonds well, but it has very little strength so don't apply over un-welded sheet metal seams or in areas of high mechanical stress (like near the hood hinges or across the joints between the gravel shield and the fenders).
I like using DuPont 999 body filler, but it's only available in gallon cans (no I didn't use the whole gallon on my roadster...really!). Your local auto body and paint supply store can provide a recommendation on good filler in quarts. Apply the filler in long strokes using a wide (~6") plastic spreader, the smoother you get it now the less sanding later. Put plenty on to ensure lows are completely filled and cover an area well beyond the area being filled(about 2x). Allow the filler to cure, but not too long. Filler ready for shaping is no longer sticky to> the touch, sands without clogging the paper and still sticks to the metal at the edges where the filler thins out. It takes 30-45 minutes in my area to get to this point (time varies widely as function of temperature and how much catalyst was added). If you wait too long (like over night), the filler gets real hard and is a bear to sand off.
Dress the bondo down with the 16" sander using 40 grit paper, this is to knock down the ridges, peaks and waves left when the filler was applied. Rough shaping and contouring can be done in this step also, don't take off too much because you need to have enough material left to get out the deep scratches left by the 40 grit paper. Next smooth out the filler using progressively finer paper: 80, 120 then 180. Note all sanding is done dry. I like to fill pin holes between the 120 and 180 grit steps. The 3M green stuff works good, but don't put it on too thick (just fill the pin holes) as this putty is lacquer based, drys much harder than bondo, and has even less strength.
Once you think you have it nice and smooth, shoot some primer over it and highlight with a guide coat. I use gray primer and highlight with mist of red primer. I would avoid top coat paints such a gloss black as I would be concerned about bonding and contamination problems (nothing worse than getting fish eyes in your final paint job). Continue sanding until all traces of guide coat are removed, you may need more green putty or gray primer during this process (I don't like to add bondo over primer, since it doesn't stick as well).
Once the damage is well hidden by the above process, shoot several coats of grey primer and guide coat again. Now sand the entire area (e.g., the door or fender) with 320 or 400 grit wet/dry paper, this sanding is done wet. Wear something you don't mind getting wet cause you will. Again sand until guide coat is gone (you are only removing 180 grit scratches now). You can use a smaller rubber sanding block now, but use long strokes. The process from a few dents to a nice body ready for paint takes only 500 short weeks, so get to it!
Good luck to all, body work is real hard work, but the results are highly rewarding.
Todd Osborn
68 2000, San Jose, CA