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
I'm just nervous about any product attacking the foam plug I made. If I had used wood to make the plug, I'd experiment with different fillers. That Piranha Puddy sounds like good stuff. I might try it on the Mach 1 to fill any voids/dents after glassing.