What about this stuff, it seems the best option so far.
TriCoPolymer VOC Free Non Toxic Lumber-Seal 1 Qt. Clear Satin Wood Sealer
There are alot of different kinds of Marine paints and epoxies. There are bottom antifouling paints.There are topside paints.Some have copper for slime prevention, some have aluminium.They have about a dozen different varnishes too.
Is there a specific brand or type that is used to waterproof a cage?