ORIGINAL: AndyKunz
A few years ago, a mold-making friend turned me on to Piranha Putty instead of Bondo. Bondo is well-known for voids and how it doesn't work well when you need to fill them. Piranha doesn't make the voids, and it sands much better than Bondo. I haven't tried comparing it weight-wise for use as a part of an airframe, but when it comes to making a quick mold it's the cat's meow.
Andy
Here's a link to the stuff Andy is talking about. I'm going to order some.
[link=http://www.autobodytoolmart.com/3m-piranha-advanced-finishing-putty-05821-p-10961.aspx]Piranha Puddy[/link]