by Roland Wilhelmy
Bondo is one brand of filled polyester resin. Bondo uses talc for filler. It is cheap and it works.
Gravel works too, but it isn't very smooth ;-). There are lots of different filler materials used with polyester resin. You can even mix your own blend from scratch (the inventor is alive and well and living in Valley Center Calif. and has written a book on the whole thing). The Eastwood Co. and practically any serious auto paint store will sell you aluminum powder-filled polyester resin, sometimes mixed with glass fiber. Most cans will list percentages of ingredients. You takes your choice. This stuff is harder, said to be stronger and waterproof, and looks vaguely like lead. I use it for rough filling, then finish off with two part glaze (a very creamy 'bondo') to get the last ripples and pinholes out.
One of the handiest tools for body work I have run across lately is a
_stud welder_. This is a special sort of spot welder that welds
special preshaped studs onto (into) the dents. Then you use a
(provided) special slide hammer or a hand pull to pull the metal to
you, while tapping around the edges of the dent to help everything
back into original shape. This is particularly useful in spots you
can't get to the back of with a hammer or spoon. Roof dents without
tearing out the headliner, parts of bent engine lids & other doors &
hoods, some nose dents, and many other spots meet this criterion.
You can put studs along a crease line and pull it out, then clip off
the studs and grind the small spots away. Cost: about the same as an
el cheapo wire welder (not MIG). If you could find a tool rental
store that had one, you would be in luck. You can approach this
technique using a MIG welder and a regular slide hammer, but the stud
welder kit is very handy and uses less heat (1 second per stud).