Hi Tom, what do you clean the casting with before you prime it for painting?
I always use IPA (Isopropynol Alcohol) which I get from work. If you can't get this from your local pharmacy you could try a local printers. I use this on resin and metal and it works for me. Another alternative is, believe it or not, cheap vodka. A mate told me about that one but I can't vouch for it as he isn't around much nowadays.
Cheers, Ron
Trying not to handle the figure as you paint it is my main recommendation - regardless of anything else this is good working practice. But it sounds like you didn't prime the metal before painting?
Washing or cleaning the casting before priming is highly recommended too, since even if there is no mould release on the parts there'll be grease transferred from your fingers during handling to remove mould lines etc.
I generally wash things in warm soapy water but sometimes clean off the castings with meths/rubbing alcohol or acetone.
Einion