I recently painted an HDPE helmet and used all sorts of primers and paints, and to be honest, depending on the material, that paint does not like sticking to the plastic.
What I ended up with was a Rustoleum Plastic Primer, a very VERY thin coat, let thart dry until it was just tacky, then 3 coats, 5 minutes apart of the Rustoleum Professional Enamel, they sell it a HD, but it leaves a nice high gloss finish, and if you wait in between coats it goes on nice and smooth.
PS, I tried at least 6 different paints, including Fusion, before I settled on this one. I was also able to get a can on Humbrol Grey for the frown and ears.
So if I understand, you're not crazy about Krylon Fusion? Your first sentence reads that way. I too am about to paint some styrene armor, and have heard good things about "fusion". Plus I hate the thought of sanding every square inch of plastic :sick
I found that the Fusion was just as good as any other paint, but considering it is primer and paint together, I could not get the proper coverage I wanted in each coat. It may be the way the spray is designed, but I seemed to be going nowhere fast., that why I opted for first priming, then painting.
To be honest, all of the paints and primers I used failed at one point or another, but the biggest problem with the Fusion was the coverage. It was however pretty easy to get off, I didn't have to use any thinner, it simply peeled right off
Also, as far as sanding goes, in my opinion you have to sand, it just helps that paint hold onto something. The HDPE had this wax like texture and nothing would stick to it if it wasn't sanded down