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.