Helmet painting advice needed

[Ducatamount]

I have painted a few helmets (rattle can flat black) with pretty good success and I am about to do another so I'm looking for any tips.
I spend a large percentage of time on prepping the helmet for paint. What is a good flexible thin width masking tape to use that can follow contours? Razor blade or X-acto knife? Stuffing the helmet with plastic shopping bags seemed like a good idea. I mask before I sand to protect the plastic vent pcs. Is this necessary?
Enamel or Lacquer (or something else)? What brand paint (rattle can)? I have read that you can spray primer (the flat black) on heavier than a finish coat of color. I saw a vid of a guy who shoots at an upward angle, spraying out the "dry line" to wet. How many coats for durability? Remember that I'm basically talking just flat (matte) black.
Thanks for any advice.

[ducpainter]

3M fine line tape is available in fractional widths and they make 2 versions...one is more flexible.

I can't help with rattle cans...I hate them, other than if you have a good autobody paint store close by most of them can load a 2K urethane in one and you can get pro quality materials in a rattle can.

If the helmet is already painted the only reason to prime, IMO, is if you had to remove decals. Just sand and shoot. That said, you can apply primer heavier than color.

[memper]

Warm cans spray better.

[Ducatamount]

A good (cheap) winter project. It takes a lot of time and prep is the most tedious part. I am not going to use the blue painters masking tape any more because the edge isn't fine enough and has slight fuzz. I will still use it to cover large areas (eye port, bottom opening).
One 15 oz. can of spray paint is plenty. Sanding (3.5 sheets of 400 grit.) and masking take quite a bit of time but you would definitely get faster the more you did it. Painting is pretty easy. You can always spray on another coat so don't go to heavy, trying to avoid runs and drips. Every time I do one I learn something. This time I put it near the fireplace to dry when I was done and woke up the next morning to find ashes had blown out and onto the helmet.  I almost had a bad spell but found the helmet was dry enough that they blew right off.  
It would have been a good job but I got anxious and tried putting on the shield before waiting long enough and put a couple of small marks in the paint but I don't really care as there will be more come riding season.
This is the third one I've done and I just realized they were all this blue color (I own nothing blue, hence the paint).
Why do I do this? I buy unused (new) helmets very cheap on craigslist even if they are the wrong color. I paid $50 for this Icon with dark and clear shields.

[1.21GW]

Just a follow up here...

Ducamount:  what type of spray paint did you use?  How has it held up on this and other helmets with respect to fading, chipping, etc.?