Would different finishing schedules inside and out spell doom for a bathroom cabinet?

Question

For a little while now, I've been thinking about making a cabinet (or something) with a finely painted exterior, but a natural finish on the inside, so that there's a little hidden gem when someone opens it and sees nicely finished wood in there.

Soon, I'll build a wall cabinet for a bathroom that I'm in the middle of remodeling right now. I'm going to paint at least the exterior to match the vanity that's already in there, and I think this painted outside/natural inside finish would be a really nice looking touch in this freshly updated bathroom if it's successful. But I'm concerned about wood movement. If there are different finish thicknesses/moisture barriers on opposite faces of the boards that make it up, I worry that uneven moisture absorption would cause the boards to warp, and the cabinet to do anything from racking to tearing itself apart. And of course, bathrooms are a particular challenge for wood movement, given their frequent, wide swings in humidity.

So is there a decent chance of success with this, or should I save this idea for a different piece, or maybe abandon it completely?

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In the past, I've had really good results painting pieces with this finishing schedule:

1. Two coats of shellac-based primer, such as BIN
2. Two or three coats (as needed) of latex paint
3. Two to four coats of brush-on water-based polyurethane

Sanding and leveling between coats.

I thought for this project, I'd try this:

1. Two coats of super-blonde, dewaxed shellac, 1 lb. cut, applied uniformly as if I were finishing like a sane person.
2. Exterior: two or three coats (as needed) of latex paint. Interior: equal number of coats of the same shellac, but this time a 2 or 3 lb. cut, to try to match the thickness of the paint coats on the exterior
3. Four coats of brush-on water-based polyurethane, or 10 million coats of wipe-on poly, also applied uniformly.

Answer 1

Would different finishing schedules inside and out spell doom for a bathroom cabinet?

Short answer, no.

It's quite normal for cabinets to be finished differently inside and out.

It's even quite common for some or all interior elements to remain unfinished. If you think about it when's the last time you saw a solid-wood chest of drawers that had any finish at all on the cabinet interior? Similarly the drawer bottoms, back and insides, and often most of the sides too¹, are also very commonly left bare as well, even if the front has a fully built up finish like French polish on an older piece or a complete lacquer job on something a little more recent.

What will matter
Material selection could end up being the most important aspect of this. One should always try to build using good, stable, stock anyway of course but it would be more critical here. Given the wide swings in humidity that the piece will be subjected to it wouldn't be a bad idea to use rift-sawn or quarter-sawn material as much as you can to minimise issues.

Obviously on top of this is construction. Just in general use trusted construction methods to put the thing together. For example if the doors will be frame-and-panel allow sufficient space for the panel to move as would normally be done, attach the top (if also solid wood) to allow for expansion and contraction, that sort of thing.

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or 10 million coats of wipe-on poly

LOL Just to mention some things that might be of value to you. You can thin less if you like, deliberately to build a little faster and if the conditions allow it. I've diluted by as little as half (1:1) and had it work fine². On top of this all of the excess does not need to be removed with wiping varnish.

It often gets overlooked with wiping varnish that it's still varnish, which dries hard, unlike many other wipe-on finishes which are blends with oils (and now waxes) where you have to be more thorough about removing excess because they dry relatively soft, and therefore can leave a surface feeling sticky or 'draggy'.

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¹ In traditional furniture, even on high-end stuff, it's quite common to finish only a narrow strip of the sides, approximately in line with the dovetails. I've always found this odd looking myself (even when done neatly, which it sometimes wasn't!) but it is what it is.

² You can actually thin even less than this and still get away with applying a varnish as wiping varnish, although I would reserve this for smaller pieces, or select easier-to-finish parts of something. The less dilute the varnish the more important it is to wipe excess off however, otherwise you lose any advantage of applying the finish in this manner.