So it's resin, isn't it? Well, for the resin ones, like the SgtFang's helmets, I have used acetone and it has worked really well, so try it. Others have used it for remove the paint of the MR helmet.
i've used acetone on roto... it will eat thin spots (say, the keyholes on the back if you've trimmed them already), but overall it's not harmful if the resin is thick. i'd use caution, but it would probably be ok.
Careful sanding should also be considered if you don't want to risk damaging the plastic, whatever you decide take your time, that is the best advice I can offer