That sounds like an awesome size for an adulte mangrove. Have not kept them in years but don't recall them getting stressed by large enclosures.
To paint melamine, your best choice is melamine paint as not much else will adhere without hard to find and nasty primers. I can't tell you much about how moisture resistant it is. My limited experience with it is that it does improve the resistance of the cheaper melamine foil stuff you find at Home Depot and other home improvement centers.
On that note, I would recommend getting a higher quality melamine like you would find at a cabinet shop. The melamine foil stuff is not very good for much other than racks.
I would also recommend sealing the floor and the first several inches of the cages walls with something more durable than melamine paint. More on that later...
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Currently keeping:
6.10 Gonyosoma oxycephala (Javan, mixed colors)
1.1 Philodryas baroni
1.0 Rhodesian Ridgeback