melamine is a pain to get anything to stick to, even after sanding. the resin they use is basically non-porous so the mechanical bond is all you’re gonna get. 3 coats might work if you really roughed it up with like 80 grit, but i’d be worried about it peeling in sheets after a few months of humidity cycles

the silicone for the seams is smart but i’d almost be tempted to just line the whole thing with pond liner or a sheet of pvc and seal those corners. way less headache than watching your varnish job slowly fail and having to tear the whole thing apart with a snake in a temporary tub

if you’re dead set on the varnish route, an adhesion promoter or a shellac-based primer would give you a better shot than just sanding. test on a scrap piece first and see if you can scratch it off with a fingernail after it cures

The melamine face is already pretty much waterproof, thats what it’s for. 60 to 75 percent humidity won’t hurt it. What swells is the raw chipboard at the cut edges, the screw holes and where the panels meet, so put your effort there.

Seal every exposed edge and screw hole, then run your silicone along the inside seams. I’d skip varnishing the faces. Poly on melamine tends to peel once it goes through humidity swings and then you’ve got flakes in the enclosure.

If you do want to coat it, rough it up and use a bonding primer first. Either way let everything cure and air out fully before the snake goes in.

tbh at those humidity levels your biggest risk is the exposed cut edges of the chipboard swelling, not the laminate faces. the silicone on the seams is the most important part. make sure you get full coverage on every raw edge and youll be fine

Why do you have to use MFC boards?

Polyurethane will peel right off laminate even if you sand it and you definitely need a bonding primer first.

I would apply a good layer of beeswax and that’s it. Pure beeswax is safe for snakes but a lot of other stuff will give off fumes. Just don’t get liquid wax that’s in solvent or mineral oil, just a plain bar of beeswax and rub it on and buff with a rag.