He's right, Bondo (it's a brand name, so I'm talking about any lightweight 2 part polyester filler) is a great product that works just like wood...
actually, found this somewhere, it's worth the read:
Auto Body Filler - Bondo
Polyester auto body filler, usually referred to as bondo, is a modelmaking tool of great versatility. Bondo is intended to be used as auto body filler, that is, to fill either holes or depressions in auto body panels in order to bring the surface back to the original shape after an accident or it has rusted out. Because of its intended purpose it was formulated to be very tough and flexible, to bond very strongly to properly prepared surfaces, to be sanded or formed relatively easily, to hold a good surface finish, and to take paint very well, yielding a potentially beautiful finish. Bondo is readily available, inexpensive, and easy to use with a little experience.
There are many different brands of auto body filler on the market and some are better than others. I have had good luck with Bondo brand and Bond Tite brand (sold at Scherba