I just asked for filler to fill alli pannels when i bought the paint for my respray a few years ago, Think it was just a bog standrard semi flexiable filler, but i was told to remove all paint from the area to be filled as it would lift slightly if it was on top of paint and crack the top coat or leave a shadow around the repair.
I followed the advice and 3 years on still cant see where the repairs were, loads and loads of scratches now could be near painty time again