Aluminium has a tendency to loose paint quite quickly as it builds up a surface patiner very quick which will make the paint not adhere properly
Good news is that there is a primer specialy made for it (self etching primer for aluminium) so if you give them a coat of that first your paint will stick a lot better
Only know this as you have to use it to spray Landrovers as the body panels are Birmabrite a type of aluminium
Most good car paint shops will have it in an aerosol can
Jason
