It's a matter of preference. Should you decide that you want to, rough sand down the resin piece before applying the fiberglass. If you're using fiberglass matting, I would suggest tearing the matting in two, thinner pieces [fillet style] and use that to layer. Just me. Good luck!
I myself have armor that both have and doesn't have carbon reinforcement.
Unless your resin helmet is really thin and is often exposed to heat, it should be fine as is. But if you feel that you need some sort of overprotection comfort, by all means, go for it. I like my helmets tough, so i then to carbon/fiberglass those without. Plus the added weight, IMHO, feels more 'real'?