I've noticed on my stormies that there's often a nasty mould line under the edge of the helmet, easy to clean off mind.
Best to spend that little bit extra cleaning them up. Also thin your paints mate both on the lining and blocking. Personally I prefer to undercoat mine grey, block up to white and save the black lining for the joints and under suit. I don't think a black undercoat works personally for these but that's just me.
