I've painted quite a few helmets. Some fiberglass, some polycarbonate and some ABS. They all get clear coated with 2k clear and I never had any problems. Fiberglass or carbon fiber are very easy and unaffected by any paints or clear coats since they are thermoset plastics. Polycarbonate and abs are thermoplastic plastics, but when well prepped, again no problem. I strictly use water based acrylic paints on helmets. Mainly Auto Air and Wicked paints. For all my helmets, I scuff with a red scotchbrite pad or 600 grit wet sand paper. I read what Mitch stated about using 1200 grit paper, but that doesn't create enough tooth for the water based paints to stick properly since they bond by mechanical means and not a chemical bond like solvent based paints. This info is provided on the Auto Air web site. For thermoplastic plastics, I use an adhesion promoter called Bulldog. Thermoplastic resins have oils and mold release built into the resin system and can be difficult for the paint to stick and the adhesion promoter resolves this issue. My first coat of paint is always Auto Air Sealer white or sealer dark. This Sealer sticks like crazy and creates a good base for subsequent layers. I also do a tape test to make sure my Sealer has bonded well to the helmet. I use a good sticky tape, and adhere it on the helmet and press it down with a squeegee, let it sit a while, then remove it and no paint should peel off. Now you also have to keep in mind the manufacturers recommendations on painting their helmet. If they state painting is not authorized and will affect the performance, then I wouldn't paint that helmet. If you have stickers on the helmet that must show to prove the helmet is certified, mask them off or if they can be removed and readhered, then do that. Here in the states, all motorcycle helmets must be "DOT" certified and the DOT sticker is supposed to show. Most manufacturers imbed these stickers under a layer of clear gel coat so they can't be removed. If they customer wants it to show, I mask it off. Honestly, most don't care and tell me to paint over it. They rather get a traffic summons than have that DOT sticker showing in the middle of their paint job. Anyway, this is the way I handle my helmet paint jobs and I've never ever had a problem. Please keep in mind each country and helmet manufacturer can have different laws governing this and I would research it first.