Wet sanding with 400 should work just as well for rattle can paint as it was for the professional paint job that was performed on my kit.
Rustoleum 2x is 2in 1 paint, the primer is built in.
Adhesion promoter is mainly used with professional painting but may work with rattle cans, contact an professional automotive paint supply for more information on that.
I've seen people use additional primers or adhesion agents and some without. Results varied, none being better then the other. Primer is great for filling in minor scratches but may not be needed with the Rustoleum 2x.
Key point is to ensure the armor is dust and water free, painted in a controlled environment to prevent foreign material or wind from effecting the process (DIY paint booth), painted in temperatures recommended by the manufacturer and following the paint process set out by the mfg.
Hope this helps