As I am in the first stages of my first build, I find I'm doing a lot of research. I've had experience with the M16 over 22 years, and owned a HK91 among others, so I'm not quite a newb. I have chosen to build a rifle in Foliage Green, which means 1) not pretending this will have anything close to a matched finish on every part, and 2) painting the upper and lower.
What to use and how to apply it is a subject with lots of different products and experience levels. One important thing I see getting right is adhesion of the color coat.
Aluminum will almost instantly develop a coat of oxidation when bare. That oxidation is what any treatment is designed to prevent, and anodizing is considered the best. (It is milspec for a reason.) Removing anodizing is a huge step backward, requires a primer to accomplish what was already done, and creates an opportunity for corrosion to creep under the primer or other mistakes to be made.
The factory answer is to anodize first, then apply paint directly to it. Architectural suppliers of aluminum products do this. Fabricators cut and bend it to shape - painted - and the color sticks without peeling. No comebacks, decades of service.
DONT TAKE OFF ANODIZING. It's the best "primer" you can get.
Can that be improved on? Yes. Use an adhesion promoter from Bulldog or Duplicolor. It helps to get paint to stick even better. After you shoot color, use a clear topcoat. Krylon and others have a flat clear topcoat that can be applied over the color coat. Just like cars, clear coat protects the color coat.
I have done that to a pricey aluminum screen door I picked up two years ago. With four teenagers banging through, the jamb and surfaces around the lock are scuff and scratch free. No scraped paint or undercolor showing.
The working commercial answer: anodize it, shoot it, clear coat it. You already have the best primer, don't mess with it.