My bike helmet somehow lost that thin plastic veneer that keeps it from looking like the goofy foam hat that it is, so I decided to give it a paint job.
Step 1: Sand and Wash
Start by removing the little pads inside the helmet. These will just get in the way or get dirty, so we won't put them back until we're done.
I don't know that sanding was absolutely necessary here, but in most instances it helps paint to adhere. I used a fine grit sanding pad to give it a quick once over and remove any glue that had previously held the plastic cover. The pad made it easier to get into the holes in the helmet, but regular sandpaper should do the trick.
Once you've smoothed out the surfaces you plan to paint, make sure you handwash the helmet and remove ALL of the dust you've just made. If there is any dust left when you paint, then you'll be painting the dust instead, and it will peel later on. A rag is especially useful to remove any remaining dust and dry the helmet. Make sure it is completely dry before moving on to the next steps.
Step 2: Get Your Tape On
Get yourself some blue painter's tape and tape over everything you don't want to paint (and I mean everything). You should also tape the inside of the helmet to keep the spray paint from getting into the interior and hitting the strap.
Generally you want to leave the whole area you plan to paint open. We're going to paint in stages, adding more tape at each stage rather than removing and re-taping a different area.
Step 3: Paint the Lightest Layer First
Now that you have your helmet all taped up, find a well-ventilated spot and lay down some newspapers. Spray ONE light layer of a plastic adherent paint and let it sit for 20-30 minutes or until it begins to feel dry to the touch. Spray a second layer and leave it to dry for at least 4-5 hours.
Step 4: More Tape
Now we're going to tape over all the areas we want to remain the color we've just sprayed. In my case, I wanted the interior grooves of my helmet to be yellow, so I taped over them and left the outer grooves exposed.
Step 5: Next Layer
After adding tape, spray the second layer the same as the first. You can do as many layers as you want, spraying progressively darker paints and taping as needed. More layers means a lot more drying time, and it might require a few days to get a good result. Once you spray the last layer, let the helmet dry for at least 24 hours before you even think about touching it.
Step 6: Remove the Tape and Admire Your New Helmet
Not perfect, but I wasn't exactly going for a professional finish on my $30 foam hat. Hopefully your taping skills are better than mine.