I worked as a heavy collision tech for several years and used my fair share of plastic fillers.I never put filler over primer, paint or anything other than bare metal. If you stop to think about it, plastic filler is only as strong as what it is bonded to. I have seen panels sustain collision damage that were repaired previously with filler bonded to metal, and the filler didn't even crack. I have seen panels with filler over paint get hit and the filler breaks or falls off the panel.I know body men who have done it both ways, but that is my experience with body filler.( my 2 cents worth)

1957 Chevrolet 1/2-ton Stepside LB in the Gallery
My Photobucket shots

The person who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself." - John Stuart Mill (1806 - 1873)