Powdercoating is probably the most durable finish you can do with those rims but it can be a bit expensive and (of course) the tires cannot be mounted during the baking process. Sharkhide is good stuff, but it does not last too long once submitted to the elements, especially if you use any kind of wheel cleaner to remove the brake dust. That'll just slowly dissolve the sharkhide away.
For the cheapest alternative, I'd recommend buying a spraycan of Spraymax 2K clear. It has to be activated just like regular automotive grade clearcoat, and is pretty durable once dry.
I'd recommend applying it as the tires are already mounted and balanced since most tire machines can -and will- roll against the lip of the rims and could quite possibly chip the clear finish off.
Potlife on that spraycan is about 48 hours, so it may be a good idea to buy 2 cans of it. Use one can for two rims, let properly dry, then activate the next can and repeat with the next two rims.
...just a suggestion though.