Spray-paint tends to be solvent-based, and spraying it onto a helmet can fundamentally compromise its structure/strength.
Please don't ever use solvent-based stuff on multi-layer plastic helmets, for sake of your brain, who needs that helmet to work properly, when one gets slammed into the asphalt/concrete.
That seems a bit unlikely. The solvents quickly evaporate off. Unless you are soaking it in solvents, there won't be enough to cause any relevant effect. Meanwhile the helmet is exposed to UV light on a daily basis, which will also degrade it over time.
You are also not going to burn from briefly touching laundry detergent, but keeping lots of it on your skin would be a problem.
Helmets are usually polycarbonate and the common solvent in spray paint is xylene. Xylene causes extreme degradation in polycarbonate. If you don't know if the helmet and the paint are compatible, it's not safe to paint it.
I'm not here to tell you what to do but most regular consumer grade helmets I've seen would be hard to spray the outside without hitting the foam especially near the vents... Foam which, once again, melts INSTANTLY when spray painted. Foam which is also basically the whole protective value of the helmet.
Anyways I'm sure there are helmets where it's possible to spray paint without damaging the foam. Or you could do a good job taping. Or you could shrug at the hopefully minor foam damage. Many helmets never have to protect your head so you might be lucky there too. All I'm saying is that there are good reasons to say "don't spray paint helmets" as general good advice.
Depends on how many vents and how small they are. At the very least, it could be very time consuming to avoid while also still spraying most of the hard plastic. Would be easier to hand-paint.
Where did i say that? Between "Don't do this, it is lethally dangerous" and "it is a great idea, you should definetely do this!" is quite a large gray area.
That's not how helmets work. The outer shell certainly helps mitigate bumps and bangs, but what really protects your head is the styrofoam inner that compresses like an airbag upon impact.
Dude there are many different helmet designs, with both thick and thin plastic covers. In a practical day to day use case, the plastic shell is there for appearance as well as protection for the primary means to prevent damage to the foam core of the helmet. The foam is no good if it gets damaged the first time you accidentally drop it on the pavement.
I'm sure it can play some role in protecting your head, but it's main purpose is to keep the foam on your head on one piece in case of an accident.