Acrylics always need a primer first.
I had that problem.Thats why I use 90% enamel paints.I use acrylics on cockpits, figures , and wheel wells,Acrylics and all other water base paints will not match enamels.
Acrylics do well on single color projects .Also on single color armor.
That is very dependant upon the surface being painted and the acrylic paint used. If painting on straight styrene (airbrushing), Gunze Aqueous or Tamiya do not need primer. If you are airbrushing over resin, white metal or brass, then primer is best used first.
Use an enamel or lacquer primer. I prefer mr surfacer 1200 cut fifty/fifty with leveling thinner. Dries faster than enamel. Just use a respirator. Never have had acrylics pull off a lacquer or emamel base.