Check this video out. The whole thing is interesting but for what we are talking about start about the 6:15 mark. https://www.youtube.com/watch?v=-yA8q4K-_Bo

That's a cool video

I can go both ways myself though strait to metal or over Epoxy I really don't have a preference Today's fillers are not fillers of 30-40 years ago.
The strength and grip or holding power of today's fillers is incredible and when combined with today's Epoxy's makes em just that much better.

The only real excuse for a failure is in incorrect application by the end user.
When used Properly as designed a good repair should out live the car.

Epoxy raises the bar in rust protection and filler longevity in my opinion.
I have just seen too much rust underneath filler that was protected by layers of paint.

i wouldnt waste the epoxy on that right now, IMO its no where near ready for that,

yes epoxy is a great foundation for filler and ultimate in rust protection, but...
but i dont believe its a good starting point for you on this car.