Crtoon83 had made a couple of posts regarding this recently. I just took a quick look but did not find them. I believe he recommended two different primers. One was oil-based, another latex. Maybe Bullzeye and Zimmer were the companies?? I don't remember.
Are you painting an existing cage or are you planning a cage from scratch? Big difference as to whether or not painting is a good idea.
Overall I tend to agree with sstorkel, the real advantage to melamine is not having to finish it.
However, I do believe it offers other advantages. For such an inexpensive product it's remarkably flat and true. Many cheaper plywoods are a bit warped and that can be difficult to deal with for the novice. Many of these plywoods also have one side that looks lousy, meaning either your interior or exterior will look lousy.
Melamine makes for a very nice looking cage exterior, even if it's one that requires some sort of additional protection on the inside. That does save a bit of money.
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Current snakes:
0.2 Gonyosoma oxycephala - (Silver/Yellow)
3.3 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)
3.3 Gonyosoma janseni - (Black & Tan)