I've used clear lacquer with success on polished aluminum, brass and copper items exposed to the weather. Admittedly, the brass and copper items still tarnish a bit if I'm less than perfect with the prep work, but the aluminum has held up well. I use thinned Deft lacquer, sprayed with an HVLP system, which shows no sign of yellowing thus far (3+ years).
For that matter, clear powdercoat would be cost-competitive with Sharkhide. And it would last quite a long time on most things, like brake levers and hubs. I have a local powdercoater that will do a batch of parts for $25. The disadvantage, of course, is that it would be a thicker film finish, and is not repairable if damaged. But if you scratch through powdercoat, you're faced with re-polishing anyway.