I've painted helmets in the past (or rather had someone paint them for me) but only when I was certain that the shell was made of something that wouldn't react with the paint. Back then the two choices were glass fibre (safe to paint) or polycarbonate plastic (not safe to paint). The solvents in the paint were the issue so whether modern water based paints would be safe is something I've wondered. If water's degrades it you probably wouldn't want to wear it in the rain.
A quick look through the pile of helmets we've accumulated shows four glass fibre ones (one of which has a sticker saying "glass fibre" on it, three that are definitely plastic and one I'm not sure about.
A few of these helmets are old enough to have their own driving licence so materials science may have moved on somewhat in the interim but certainly with plastic helmets not only was it no paint, it was also no stickers (the adhesive could supposedly weaken the plastic) and not much sunlight either to avoid UV degradation.
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