how can you say "prep is everything" following the fact that you did zero prep work? 
don't get me wrong, I'm happy for you that it worked out in your case, but I gotta say I think you're probably the exception to the rule.
when I did mine I bought a tube of black silicone to re-seal them, but I think I only ended up needing it in one little spot where the old stuff totally pulled apart
so...those rings don't come off for the painting? I assumed they could be disassembled
I didn't prep as in..sand or prime. I did prep by painting the whole casing white, then by taping off those circles (no they don't come off, its one piece) and painted the rest black. so the "prep" in my case was just taping off what I didn't want black...very, very carefully I might add.
I should say I actually started to sand at first. Luckily my neighbor came outside and saw what I was doing. He made a very good point (and he paints cars for a living so i was inclined to trust him)
You have to remember, for the most part nothing has touched the plastic inside of the light sense it was made. There is no dust or debris in your lights, and its already a very smooth surface, so why sand?. Also the krylon pain is made for plastics without priming..so why prime? Im definitely not trying to sway anyone from sanding and priming, Hell it might look better, (i have nothing to compare mine too) just an idea to help save some time..
In the end tho, you may be right i could be the exception to the rule, i have only done my headlight so i don't have too much experience to back up my theories, all the more reason to help someone else right?
