I presume that your beams are from anodized aluminium. With the result that the upper layer of the aluminium is harderned (try it to manufacture with a lathe and you will find out how hard it has become)

I remember me once having painted a alu H14 mast with DD paint (2 comp PU),simply after having the mastsurface lightly sanded. No primer. It lasted for ever.

Ofcourse very carefull degreasing with aceton is a must.