powdercoating requires 400 degrees for 5 mins followed by 350 degrees for 15 mins. head over to autozone/pepboys/whatever and read a can of bondo to see if it's tempurature resistant. i don't know off hand, but would tend to doubt it.
another issue is that the filler likely isn't conductive the way steel is so the powder won't "adhere" to it during the spray process. when sprayed, powder is emitted as a positively charged dust-cloud which is attracted to the negatively charged METAL part.