I have used Dio-sol Barrier and/or thin coats of an automotive lacquer based primer/sealer to protect plastic from being eaten by lacquer paints. Lacquer based Dio-sol is getting harder to find, but if you can find it their primer works very well as a protective coating. Aply it in thin coats. It sands nicely.
The best results I have gotten with Alclad has been with a gloss black acrylic as a base coat. I used it to put a chrome finish on a set of car bumpers and got great results. I'm going to practice and experiment with it a little longer before I try it on an airplane.
That B-29 looks great. rangerj