We do direct print with UV - Never tried on painted aluminum... Mostly we do it on ACM, Coro, Occasionally wood, and plastics. We printed a little punisher Dibond skull to test a print and cut when we first got our cutter - We used a bonding agent and everything, It was mounted indoors... and it started to peel after 3 years. No exposure to sunlight... It could be because it was placed on a Cutter and constantly moving / vibrating, but 3 years with no sunlight exposure kind of annoyed us.
Materials like ACM / Metal don't bond too good. It's a solid, so the ink doesn't seep into the material like a solvent printer on vinyl will... I don't think you'll ever get even near the amount of time you'd like with UV. It's a good technology and it comes in handy, but for the length of time you want it to last... I think your only option is Screen print, or using a high quality cast material with laminating.
Come to think of it... We just did a huge job for the local port/ferrys, We screen printed directly to powder coated aluminum. With a 7-10 year guarantee - It's a repeat job that we've done for years and years. That was mostly white ink on a blue powder coat though.
I don't know enough about screen printing to comment on what you need to do for direct to aluminum though. I work in the digital department, never actually done any screen printing myself, it's always a co-worker. Gino's been in the game for many years, and he knows what he's talking about - I'd suggest following his advice if you want to do a direct print. I think with the right ink, you may be fine.
Do you do your own screen printing, or plan on picking up a system to do this yourself? Or are you looking for a supplier? If you're going to outsource... I'd contact suppliers through the merchant program on here... Ask them what materials/inks they use, how long it lasts, etc.