So, I finished the helmet.
Found a different white (radiator white), sanded it down a bit and painted over. The only thing that rubs me a bit is that I wash't able to remove all the paint and start over fresh, so in the end, there were quite a few layers of white and gloss varnish, partly obscuring the nice bumpy detail of the helmet, and not making it as crisp as I ideally would have wanted it.
The best would have been to have only the thinnest necessary layers of paint, to retain the texture as much as possible, but I think it turned out awesome anyway, and most of the bumps and irregularities are still very visible.
And in the end I managed to get a pretty good match in colour to the 2mm TM I already have.
All details were hand painted by myself.
Here come some medium quality pics with indoor lighting, will take some better pictures soon.
TM on the left, RS on the right:
Bonus picture of me wearing the helmet, I'm the trooper on the left:
I guess the next project will be getting a "HDPE" helmet...
It never ends...