Belgom Alu is popular but I'm not that big of a fan, I refurbish and polish a fair few split rims and I think the best thing you can use is a good quality automotive paint sealant. I use Chemical Guys Jetseal which isn't cheap but it buys me about 2 weeks in bad weather before any water marks start appearing, and as a bonus any dirt will just hose off.
Here's some wheels that won me best of class a few years ago…
Although after saying that I'm thinking about using some of this on the wheels I'm building at the moment, although I'm a bit dubious, I bet it ends up chipping and then you might as well rip the lot off….
Lovely finish on those bits btw, what are you using for the final stage? I usually use a loose G with with blue.