I sanded down the entire surface just enough to get below the surface level of the paint job.
Started with an 400 grit sandpaper, then went over it again with 800 then 1000.
It would be a good idea if you can remove all inside components AND the visor before you start on it. If you bitch that up, like I did, you have to buy a new one. They're out their, but it's a pain in the arse. Thankfully, my replacement ended up looking actually more screen accurate, but the Hasbro one isn't bad.
Thanks! I use those same levels of grit for gundam & bandai star wars models, interesting to see they'd get good results on this helmet. I think for what it is, and the price, the black series helmet is great already but can be improved with an actual paint job. Ultimately I'd like the new RS helmet unpainted as well maybe something like the bobamaker that I've painted myself. Planning on painting the black series as a tester.