ok please dont hold me to this because Im not sure how it works. So dont go use this on ur brand new helmet u just painted. this is only a suggestion and im not sure if it will work or if will have a side effect.
When i was making my right gaunt out of thin pieces of pine wood and syntra i noticed that there were huge gaps in the wood and syntra and the wood's grain was still showing thru the paint. SO i got myself a can of chrystal clear enamel (triple thick), the kind that you use to coat a table or something. I used the entire can on it and it looks pretty much like on solid block of plastic now, as opposed to many different pieces.
I guess what u could do is put a few very light coats on ur helmet, the enamel MAY fill the brush streaks in and give it a more smooth look. However Im not sure, so I dont want you to mess ur helmet up because of what Im saying, so do this at ur own risk I guess.