In my airplane paint shop, we always put the bondo on bare aluminum (no primer but after alodined). Mostly because it is the fastest and easiest way for a production shop. Otherwise the sanding will go through primer anyway and need more work.
The finished bondo area could use some extra spot primer coats first, then prime the whole thing.

When young, I worked in an airplane paint shop that never used primer at all, just etched and alodined the stripped aluminum and coated directly with white Imron paint. This saves weight, cost, time, and is much smoother than primer. Makes it easier to strip later also. A working commercial plane gets scratched more than corrodes on the external skin and a new paint job every 5 years is needed anyway. Maybe 10 years, if treated well.
I think only float planes need primer, but it is your call.