For PE, or any metal for that matter, I use an etch primer in my airbrush. There are two component types and one component. They spray extremely thin, you could see the metal shimmering through, but that's enough. I use it on my brass, nickel silver and white metal models. I used it on the PE parts of the Fokker D.VII. I was amazed at the fact that although I had to give fairly sharp bends to the PE safety belts, they paint didn't come off.
I order mine at Phildie's in the Netherlands, but I am sure it is available in any country. Just try and find etch primer.
Another possibility which works fine too is to use the primers in spraycans used for car repair. The best brand in the Netherlands is Motip, it may be sold under another brand name abroad. These are mostly acrylic type, but accept all kinds of different paints on top. I didn't have any problems with that using enamels or lacquers based on nitrocellulose. The last type is my favourite for metals, but can be tricky, because sometimes it attacks some plastics. Nice about nitrocellulose is that it dries extremely fast and you can put one layer on top of the other in no time.
regards,
Ivo