How do I paint ALUMINIUM?

Hi, Bikers question please. I have a double plated aluminium sump guard on the bike, which as it is always filthy and looks naf, I decided to paint it black. Having first cleaned and degreased the inner plate I painted it with hamerite enamel paint. Spot on, 1st part done. Same process for the second lower plate, upper side OK, lower side that is visible, bubbling streaking (not me!) sagging, YUK. Scd it all off, cleaned it up, painted it again, Naff, just the same. Any suggestion would be most helpful and much appreciated, thank, Andy Pandy.