One of the key things you need to do with virtually all Forgeworld models is to give them a good wash.
When the Forgeworld models are cast, the mould is given a spray (I guess) of some kind of lubricant to allow the cast model to be released from the mould easily.
However the lubricant also acts as a barrier to paint, so as happened with previous models I (and others) have painted is that the paint flecks off.
Washing the model in water with a drop of washing up liquid should remove the lubricant.
Avoid using hot water as this could warp the resin (a useful tip if you need to warped resin back to its original shape).
Once washed the model is then ready for painting.