Best Filler for Coving Cracks?

I'm fixing some small-ish cracks in an Edwardian coving. The cracks are only 1 or 2 mms wide and most are only a couple of mms deep (1 or 2 are a bit worse). It seems to be made of a very fine white substance (Plaster of Paris?). A couple of questions:

1. What's the best thing to fill it with in terms of ease of filling and likely longevity (esp flexibility): standard Polyfilla, fine filler, Painter's Mate, Plaster of Paris, that new very lightweight filler?

1. Should I prime the cracks with anything before filling?

Thanks,

Michael