Clear anodizing, its actually really common, most people with aftermarket rims have it and just don't know it. Bare aluminum oxidizes. The way rim manufactures get around that (the good ones anyways) for their polished aluminum lips, is by clear anodizing them. Its not 100 percent clear, it leaves a sort of pearly tinge to them. Hard to describe.
Heres a good side by side comparsion pic I just snapped...
once its put into use and left out in the elements for a few weeks or months.
If your aftermarket rims have "polished" lips and have never needed to be polished to restore their shine, then they are anodized clear. All of the Compomotives and original finish Enkies I've seen have it, its very common. You can actually wetsand it off too, not that you'd want to unless the rims are in really rough shape. Anyways, its fairly expensive to have done, costs a bit more then powder coating typiclly. Its also a little hard to find places that do it. Places that do gold plating, chroming and normal anodizing are your best bet. It aslo invloves alot of heat, so unless you have 3 piece rims and you take them apart, you have to strip off every bit of paint on your wheels before it can be done. It was suggested to me by Modern Autoplating (local company that does it) that powdercoating MAY survive the process, I've yet to test that theory. With powdercoating there is a special tape they have that can survive the heat and can be used to mask off surfaces. Say if someone was adventerous (and thrifty) they could have the black lines on the ptype wheels powder coated black, then clear anodize the rest. But a more econimcal way to go would be to have the whole wheel powder coated clear after the lines were done.
I wouldn't recommend clear anodizing wheels myself. Clear anodizing is usually pretty dull and IMHO is a hard thing to keep clean. Makes sense because anodizing is actually aluminum oxide (Al02 i.e. oxidation). Anodizing is actually one of the cheapest finishes you can do on aluminum. I usually spec my AL parts in anodizing when I can on non-process/cosmetic part at work for cost reasons. I don't know how exactly you would expect anything other than the aluminum to survive the anodizing process. Depending if you do type 1 or 2 anodizing (type 1 is sorta out of date these days), the part is getting dipped in either Sulfuric or Chromic Acid. You can actually have multicolored anoziding done if you want. I've seen samples they've done with graphics done in anodizing. So you could probably have the black stripes anodized with the clear. Got me how much it costs though, since we've never had any done.
Depending on where you are at you can run with uncoated Al for a while. I polish my wheels about once a year. I ran about 4 or 5 years on polished stockers in my 1st MkII (roughly 100k miles or so) and they still looked new. You just have to keep them clean and away from salts and the like.
I think it must depend on the aluminum to a certain degree or something. Some aluminum wheels fade so quickly. For instance, with Z car 14x6 6 spokes, I've friends with them and theres a very noticable loss of shiniess after just a single week after being polished. Theres a few tricks to making them last longer, like waxing them, but I think some aluminum must oxidize slower. Those wheels on the right in my first pic have been polished for quite some time and still look really good, mind you they've been indoors the whole time too. I prefer the look of polished aluminum over clear anodized aluminum, but it hardly looks bad. Like I said, most aftermarket rims come that way and they look great. And on my anodized rims, they're much easier to clean then bare aluminum ones. Not nearly as easy as painted rims though (or even better, powder coated).