I've had excellent results on several stocks and various AR parts using proper surface prep, either Krylon or Rustoleum, and topping with Rustoleum matte clear. If you topcoat with Duracoat matte clear I would expect the end result to hold up nearly the same as if you had used Duracoat for all coats.
I haven't done a Manners, but on various plastics I've done my surface prep with a couple cycles of household cleaner/degreaser (Lysol and a knockoff brand) followed by rinsing with HOT tap water until surface feels completely free of grease or detergent, then letting it air dry and handling it as little as possible so the oils in my skin don't contaminate it. YMMV but I have not had any adhesion problems at all - the only finish wear I've experienced has been where metal parts (AR charging handle, sling loop) were heavily rubbing on the paint. Follow the paint instructions carefully including dry and recoat times, and not recoating outside the window for the paint you're using.
