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.
I have found the casting to be variable good to awful so tend to clean them up to what I am happy with. My painting style is get them done till I am happy with them or I tend to get bored and start something else. I know you should but I never thin my paints or mix colours as it takes away the little time I have
Lining is something I can't do as my eyesight is crap and my hand shakes like the devil
I'm hoping to get enough painted up to them motivate me to do some scenery