
Hmmm I wouldn't paint it...the very nature of what it is made of and for should denote some measure of sealant on the surface of the mask...but if you really want to, then clean and prep the surface with a proper residue free solvent and prime with a rubber/plastic friendly paint.
I might be tempted by this gas mask to cut the upper section away, exposing the eyes and forehead.
The eyes are key to a good costume in most cases...you can show so much with the eyes and you lose so much without them IMO.
