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I want to put shellac on the sides and bottom of a cutting board made from mesquite to protect it. Then on the top (the cutting surface) I was planning on using mineral oil.

I did a fair amount of searching but can't find any references to anybody else doing something like this. i.e. a seal on sides/bottom and a non-sealing oil on top.

What issues could I run into with this?

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    Nah, don't do this. Shellac is famous for how little protection it provides from water, and TBH it's just not that tough. It's great stuff for what it's used for (like many woodworkers I'd be lost without shellac) but this isn't a suitable application for it. Anyway that aside, what makes you think the mesquite needs protection? I mean literally, at all. I have a pine cutting board — yes, pine — that's gotta be something like 5 times softer than mesquite and despite being older than many member here it's holding up to daily use and scrubbing with boiling water just fine. – Graphus Sep 27 at 8:01
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    I have used a mixture of food/medical grade mineral oil and beeswax (4:1 ratio) recoating it whenever the surface dries out and I want to restore color. As Graphus points out a board can get along fine without any finish for decades. – Ashlar Sep 27 at 15:05
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    "What would be a more appropriate finish if I wanted to seal and protect the bottom/sides? " They don't need sealing (or 'sealing' — as it's commonly used it doesn't mean what it says) + they don't need more protection than the top gets. While it is possible to completely waterproof wood (e.g. boats) you wouldn't want to do this here because the top will get wet, so you'll get differential swelling and drying, top v bottom. This is the very thing that makes boards that sit on a puddle of water bow, and sometimes crack (occasionally catastrophically, literally the board is a write-off). – Graphus Sep 27 at 18:43
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    I forgot to check one thing, is this a long-grain board or built up from small pieces, with end-grain working surfaces? I've been presuming that this is a long-grain board you're working with here, rather than butcher block. – Graphus Sep 28 at 6:03
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    Glad I surmised (haha, guessed) correctly ^_^ In that case my comments above hold. If this were butcher block it can be treated very differently because end grain is very absorbent. Long-grain surfaces aren't, and penetration of any sort of finishing material is surprisingly shallow. – Graphus Sep 28 at 15:22
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What issues could I run into with this?

Mainly, you'll run into a not-very-durable finish on the sides and bottom, and possibly a weird transition between the oiled part and the finished part. Water and other liquids will likely seep under the shellac film where the sides meet the top. Water will no doubt contact the sides and bottom of the board and cause the shellac to cloud. Mechanical damage from knives and other kitchen implements will scratch or chip the shellac.

I did a fair amount of searching but can't find any references to anybody else doing something like this. i.e. a seal on sides/bottom and a non-sealing oil on top.

That should be a strong hint that it's not a plan worth pursuing.

I'd guess you're thinking of shellac because it's food safe. But although shellac is safe to consume, it's not a good finish for standing up to the kind of abuse that cutting boards get.

A much better plan would be to finish the whole board with food grade mineral oil, butcher block oil, etc. You'll need to re-apply it from time to time as the oil soaks in or is washed away, but it's not something you need to do often. Even with your hybrid oil/shellac proposal, you'd still need to periodically oil the top; using oil on the entire project won't increase that time much at all since the bottom and sides likely won't need more than a swipe or two with the same rag.

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