Never rush!
Don't try to paint the whole helmet in one sitting, do a small section, from a foot to two feet away, let it dry completely then do the next section. after that, do the same fine sanding method as you said above and apply another layer again, in sections, keep repeating... I used to paint guitars as a hobby and this is the same teqnique I used, you always get a very smooth and even finish that even Fender would be proud of.
I agree with Baddblood if you want a perfectly smooth surface to start with.
I sanded,lightly, the primer on my helmet before adding the silver, But I didn't want a perfectly smooth surface.
I left a few imperfections, if you will, in mine. Makes it look more beat up.
If you study the CD Ref, the Actual helmets have dings and scratches and little "Flubs" in them.
So, yes sand, to give the silver something to bite to, but how much is up to you.
Just my 2 Republic Credits.
Thanks,
Robert E.