Everyone starts somewhere. But start right. If you aren't an artist, you have no business doing this stuff. And if you don't want to pay attention to the techie stuff, quit now. Following is a brief explanation of what is involved in prep'ing and painting.
Before I intrude on your technical discussion here, I'll give you some background. I've been doing painting of this type, on helmets, goalmasks, racecars, conversion vans, motorcycles, etc. for about 20 years.
In that time I have seen quite a few do-it-yourself jobs come my way ... for well, restoration.
Invariably, people use the wrong techniques, the wrong paints, the wrong application methods and then cover it up with the wrong sealant.
Then, when I get to fix it, I can charge double the amount I would have, had they come to me in the first place.
IF you are going to tackle this yourself:
1. remove face shield, etc. - anything that you can get off without damaging it.
2. if the original surface is undamaged, you can "sand" it using a scotchbrite pad (the industrial type sold to body shops) or 600 *wet*, not dry, grit automotive refinishing sandpaper. Chips, deep scratches, etc, should be filled with autobody repair putty and sanded smooth. You may be able to sand out a scratch, but you run the risk of creating a flat spot that will show up later.
3. mask off all areas not being painted with (good quality!) masking tape and wax finish paper. Take care not to overmask or undermask. Details may require trimming with an exacto knife. On a project as small as a bike helmet, details become very very noticable.
4. equipment required: a decent airbrush, filtered air line with minimum 60 lbs pressure, dust free environment with exhaust fan...(I might as well just come out and say "paint booth"...)
5. design details:
a)if the design has hard edge elements, such as stripes or lettering, you will need to lay these out with tape or suitable masking material, and probably cut them out with an exacto knife. If multiple colors are planned, plan on double masking. Do any markings on the tape!
b)if the design is free-form, then your airbrush is all you need. Never you use any pencils or markers for preliminary layout on the surface - doing so contaminates it. Your airbrush is your drawing device.
6. Paint: Automotive base-coat/clear-coat system. There is no substitute. Previous paint systems (lacquers, acrylic enamel) are obsolete. Remember that different manufacturers have slightly differing mix rates and additives.
7. clean the surface with wax and grease remover, a tack cloth and then paint the thing. You may need anti-stat in some cases, as well.
8. Paintbrushes are verbotten.
9. Avoid touching the paint surface with your fingers at any time. More contamination. Don't spend a week at it either. If you can't get it done in one day, you took on the wrong project.
10. Within 24 hours clear coating must take place. Clearing MUST be done wearing suitable protective wear in an inspected and approved body shop paint booth. MUST. Isocyanates are bad for you. (*btw - 10 coats are NOT better than one.)
When clear coat is dry (several hours) reassemble and enjoy.