[Ron Ramirez]

You can put Bondo down on bare metal. The chassis in question is, by electroubleshooter's own description, already rough, so adherence should not be a problem.

Two examples of this being done in automotive work:
http://www.purplesagetradingpost.com/sum...aint1.html
http://www.a2zautoforums.com/showthread.php?t=936

I've done a little bodywork before, and I will be the first to agree that Bondo is not good for deep, structural repairs (i.e. large holes or dents). Nevertheless, several years ago, I owned a 1974 Oldsmobile Custom Cruiser with rust around the wheel openings. Yes, I filled the holes with Bondo, using fiberglass sheets as a backing, then primed and painted. Surprisingly, it held up - and much better than I ever expected it to; the stuff was still adhering when the car's engine threw a rod a couple years later, and I sold the car for scrap.

A radio chassis is not going to be subjected to the extremes of an automobile; it will remain indoors in a controlled environment. Therefore, flaking and peeling should not be an issue, and either Bondo or the spot putty would be O.K. in this application.