There is direct to metal clearcoat. I used one by PPG on a show car a few years ago. However, it was to normal body panels not aluminum. The prep process for aluminum is a little different but that being said, I would be very surprised if there wasn't a dtm for aluminum as well.
Just as a side question - what do people think about vinyl wrap now? I hated it for a long time because I thought it looked really cheap, but now it seems most people can't tell the difference. I'm still going to real carbon fiber my car (I already bought it, lol) , but I have to admit it is very tempting to just vinyl wrap instead for the price difference and ease of use.