I suppose it depends on what filler is being used... if reinforced filler is being used the metal would need to be roughened up before application otherwise it would not adhere properly.
Im not sure about the 'easy-sanding' fillers applied over primer, with my short stint at Berlina Bodyworks we always applied fillers to bare metal, and there were several Ferraris in the shop at any given time. You may find it interesting that many pre- 80's Ferraris were completely covered in filler at the factory before application of primer and paint. That’s why so many unrestored 308’s look a little bit wavy across the flanks.
I would be worried about fillers other than Glaze (also referred to as niki in Australia) adhering to a primed surface – but it would be interesting to find out more about this technique?
Awesome write up by the way!!
