Hey Clutch! I used a grey enamel undercoat. For the white, I had a tin of some rather old, original paint for an Holden Torana lying around, and used it. Its an automotive acrylic (not 2pac stuff). It was a dream to apply and came up with a beautiful gloss finish. I know I'll be doing all my helmets with acrylic from now on.
Pretty sure you won't find this stuff in the US though, seeing as the Holden Torana was only available in Australia. However you should be able to get those auto 'Touch up' paints in the US. They come in 150, and 400g spray cans, and should be just as good. If not better. I painted a vader with it some time ago, and I was stunned with the results. I can get a pic of one of those cans if you'd like.. I'll have to do it when I get home though.
I have heard some horrible stories about krylon paint, and HDPE. The paint comming off in big sheets, and flaking off just with handling. Not the way to go IMO, and the finish just looks dull unless you polish it up like mad. Even then it looses shine after a while. All the pics I've seen of ANH trooper helmets, they are glossy as! Including the sandtroopers. I'm pretty sure AA would have used this kinda stuff (auto acrylic) on the originals... The acrylic sticks really well to the plastic, and deffinately wont chip with normal handling (unless your on a movie set). Trooping, I wouldn't know? It would take a fair hit, with a somewhat pointy object (like a bench corner or something) to chip it. It also depends on how thin the coat of paint is. The only places it wanted to naturally chip was around the ears, and underneath the neck trim. The same places you see the originals chipping.. they're the places with the most flex, and least coverage. Even then the chipping was only minor.
I actually tried painting it first with 'Dulux' (or 'Dulon'? I forget what it was called) housepaint. But once it was thin enough to shoot through a spray gun, it lost all of its adhesiveness (is that a word?) and ran straight off leaving only the grey undercoat behind. No way they painted them with housepaint, unless it was with a brush. I had to sand the whole thing back to remove any residue left behind.. The end result was well worth it tho.
While I'm at it. For the weathering (which doesn't show up in these pics) I used a little off cut of some rubber neck trim. I smudged it where I wanted the weathering, then rubbed it with my sleeve to soften it up a bit. Leaves it looking dirty and USED!! I'm very happy with the results
I hope that helps some ppl out. If there are anymore questions, I'm only glad to help.