I'm going to be painting mine with automotive enamels. It's not authentic, but it definitely will work for durability. It's always a compromise on some things between authenticity and modern convenience. For me, it's the ease of maintenance when I'm using my helm for rebated steel combat. I figure I wax my helmet twice a year just like my truck, and it should hold up okay.
Also, it helps to do a more authentic liner, at least in style for the helm too. That way you can take it out and maintain the inside of the helm easier. All our sweat and such can wreak havoc on the insides.
I'm debating on using more authentic techniques for my living history helm, I dunno. We'll see, hopefully this week I'll get pics from Les of my helms!