It's pretty normal, it's not too realistic of a metallic paint, which is good because the original prop's isn't too good either.
It's why cold cast helmets often look a little weird, because their metallics are too shiney and real.
Only the real metal parts like the borden and ears should have a realistic metal shine to em. Rest just that sorta dull slightly-reflective aluminum look that Silver 11 gives.
That said if it's coming out super dull, could be a thinning or humidity issue making your layers come out a little grainy. Micro textures like that will naturally create a more matted appearance. If that is the case you can try steel wolling it a little to smooth it out and re-layering. But I wouldn't fret to much about it, as said, it's supposed to be a weak effect, and yeah, any un-masked clear coat will kill it completely anyway.