plywood rack build question about sealing wood

I am building my first rack and chose to use maple plywood instead of the usual melamine.

I plan on painting with a semi gloss paint and seal that with minwax polycrylic.

Can I get away with just sealing the top where the majority of the humidity would be or should I seal the whole inside of the rack?

Thanks

If your going to take the time to build a rack, I would take the time to make it last as long as possible. Theoretically, you shouldn't have to, but I would. I've built several racks and cages and sealed every inch multiple times. They are all just as good as the day I built them. At the very least I would out two or three costs and several on the underside where the humidity will gather. I've made all of mine out of birch faced plywood and they all came out great.

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Without being sealed with at least 2 coats of non-toxic clear coat your rack with eventually succumb to moisture and begin to weaken. That is part of the reason people use melamine. It has the white plastic coating and the raw edges are sealed up by the nice white iron on tape.

Most plywoods are very moisture resistant (interior and exterior). Many will stand up to full submersion, dry cycles into the 100s of times. Melamine is not water resistant at all, and needs to be fully encapsulated every joint sealed before gluing and after, no water can get in and no gas should escape. It is used because it is cheap, the same reason why it is used for cheap furniture.

Yes plywood is best sealed up it looks better and is easier to clean it prevents stains it is not however about a moisture barrier. This is why it is fine to use water base products (permeable) The toughest coatings and most complicated are two part LPs and epoxies they are close to full barriers. One part polyurethane is the hardest of the one part coatings (varithane for example) Next would be oil based coatings they often are very durable deck products and bathroom products should be avoided (often contain toxins to prevent mould or insects.) The same can be said for 'spar' varnishes they typically are soft to be used on flexible wood (boat spars! AKA masts ect.) they never fully harden and wear poorly. There are a handful of hybrid coatings modified latex ect. they can be hard wearing as well. Then the true latex water based coatings are last they are not as hard wearing as the others but offer easier use and lower toxic compounds when working with them. All coatings need enough time to fully cure and out gas. Most oil based can be baked out with heat and lots of air flow epoxy and LP need at least a week in the correct temp for that product (huge variations) if you have never worked with LP or epoxies they can be very tricky and need some practice (and cost lots so it is hard to get experience).

Thanks for the quick replies. I will seal everything up with a couple coats and a few extra on the tops

I've always used a good spar varnish to finish my racks or cages with. The stuff is designed to be used on boats. It's resistent to cracking and moisture and heat so it's always worked really well for my snake enclosures.