-Manufacturer do not recommend painting helmets.
Here's the one you can read the most about out there. If your helmet is good quality, fiberglass, carbon, kevlar helmet, there is absolutely no problem at all at painting it. You just need to follow some rules and be careful about it and the helmet integrity and safety will not be affected. The steps we take as responsible helmet painters will ensure your helmet will remain safe for years to come. Beware of overnight painters and garage airbrushers who say they can do the job. It is a risky thing to play with your head like that.
-Fomula One drivers go through a new helmet every race.
I have no idea about other company but, a company I know well are giving away 8-10 helmets a year to F1 drivers to wear. They may get more painted for sponsors duties or gifts. In the past F1 drivers used to get more helmets but, with the arrival of the $5000 carbon fiber helmets, companies are less willing to give them away.
-F1 helmets are made to measure to the driver's head.
Absolutely false. The helmets needs to pass the homologation tests and one example of each sized helmet used in F1 pass the homologation. If a new driver arrives in F1 and his size of helmet is not yet homologated, the helmet manufacturer has to get it done before he can use it in F1. For a complete list of homologated helmet for formula one, including sizes available, follow the link: http://argent.fia.com/web/fia-public.nsf/87488537EEBCC8D4C125746A00540772/$FILE/L33_Approved_helmet.pdf. Other top series drivers in the world including IndyCar, DTM, Formula 3 drivers, use regular, off the shelf helmets. These may be modified a bit to suit their specific need and head shape.
If you read this and want to add more questionning or have a better answer to those questions, feel free to contact me. I don't know everything there is to know about helmets...