UPDATE:
i used light sandpaper to clean well all the surface, then the guy at the shop suggested me to use denatured alcohol first instead of antisiliconic solvent (which i'm gonna buy anyway this afternoon). since i already had my bottle of denatured alcohol at home i decided to give it a try and i cleaned all the helmet with it. then i used water and soap, let it dry, and i used the antistatic towel to finish the job.
i repeated the whole steps 2 times.
at the end i didn't notice much difference on the surface which is still smooth, is that ok?
i mean i know that paint needs to be sprayed on a rough surface (at least a lil bit) in order to get a good result. maybe it's rough at a microscopic level and i can't feel it lol.
if required im gonna use a lil bit of antisiliconic solvent and then primer, sandpaper, and primer again and finally paint. i think it'll work