Bjorn, Go over them with a scotch-brite pad or fine (320 grit) wet or dry paper to get rid of any surface corrosion and to give the primer a "tooth" to hold on to. Wipe the panels down with a dry cloth but don't wash or rinse them as that will encourage them to start corroding again (bare aluminum is reactive enough that it starts to form surface corrosion in about .07 of a second). If you can find it, a zinc chromate primer is good to use; the zinc is there to act as a sacrificial element to protect the aluminum. Once the zinc chromate primer is thoroughly cured hard and no longer feels tacky to the touch, paint and label as desired.