Well, it should be 5083 Aluminum, so why paint it at all? If you have bare metal, just leave it, or Alodine it. If you already have paint and are trying to make it pretty again, there are a couple of ways to go. There is something like Alumigrip which will use an etching primer and a linear polyurethane type top coat. Or you can go with a urethane rubber type product, or an epoxy paint.
After further reasearch I think Chris Craft used it for a sound deadining. My 41' aluminum has a rough hard surface in the bildge. I looked at one 15 years ago and it had a soft surface that you could scrape of to get to the hull.
