Two pot polyurethane is generally the gold standard, however there are some acrylics and even water based systems.
What is just as important is the surface preparation and primer.
I did alumiprep plus alodine. I followed that with two part sherwin williams Strontium Chromate aircraft primer. This is the gold standard and the primer is about the same as Boeing (BMS 10 -11?). The primer is about $100 per liter.
Disimilar metals should have faying surfaces sealed with AC240 B something polysulphide rubber sealer. This used to be called PRC 1422 G8 and is commonly called fuel tank sealer. The usual name is “brown #$@%” once you’ve used it you know why.
In the old days not many light aircraft used these systems - hence SIDS.
Weight is a consideration.
Top coat is Valspar two pack automotive.