We are just using mineral oil around the prep sink with our walnut tops at this time. I used marine grade finish around the clean-up sink, both are undermount. Mineral oil, while water resistant, isn't waterproof. Even after daily oiling and weekly oiling, water will absorb. Beeswax added to the mineral oil gives me a bit more time to clean up. In all likelihood, I'll coat the area around the prep sink with shellac and then do an oil based finish of some sort. If you were religious about wiping up water around the sink and used an oil/wax combo, you'd probably get a lot longer life out of the tops.

FWIW, Pugler, we used Waterlox, a much more durable and water-resistant finish, and, since the sink area is "temporary" counter, set the undermount on top for now. As it is, though, we should have put additional coats in the hole for the faucet. There is a thin discolored ring around its edge that would make me uneasy about what we'd have if we had undermounted the sink. Certainly, I doubt mineral oil would have cut it long term without exceptional care.

FWIW, our really noticeable problem is where the soap dispenser rises directly from the wood. The soap chemicals from occasional drips and drops not wiped up (DH the big culprit) started penetrating the finish fairly early on.

Note that this is mostly a maintenance problem and could have been avoided, or greatly minimized, with greater care, the kind someone using mineral oil might do as a matter of course. Plus, mineral oil can be reapplied very quickly and easily, unlike Waterlox, whose fumes require relocating for a couple days and can't be kept stored for touchups for any length of time. Other options would have been a metal base plate (yuck), or, better choice for us, waiting to build one in because we've always planned to wrap this high-maintenance area with stainless steel eventually anyway.

Also BTW, Waterlox has been beautiful and durable on all our non-soap/water counter areas.

rosie brings up a good point. Soap will cut through the mineral oil rather quickly. We don't have a soap dispenser at the prep sink yet, but I suspect any soap drips would cause issues almost immediately. I do keep a rag in a ziplock saturated in mineral oil in the kitchen at all times. If it looks dry or I've rolled out dough, I always hit the area and let it soak while I clean up the rest or leave it on overnight. I've found that no matter how much oil I put on, it will always soak in where there are knotholes and sapwood. I coated the underside of the counter daily before installing and then the top daily for about 10 days, and then every few days, and then once a week. When we cut out the sink hole, the oil had penetrated about 1/4" on the top. Maybe 1/8" or so on the bottom.

We've had butcherblock (Perfect Plank, not IKEA) on the coffee bar for a while, but it was treated for maximum water resistance.

It was very thoroughly finished with Waterlox - two coats on the bottom of the piece, 4 or more on the top, and on the cut ends, AND when the opening for the sink was cut, I thoroughly coated those cut edges a couple of times.

Then the sink installer put a full-coverage layer of silicon caulk around the cut edge and for an inch or so around it on the bottom and let it dry and set the sink in with even MORE caulk.

It has endured casual treatment - many coffee and water spills which aren't always wiped up immediately - and still looks good.

We did the same around the main sink.

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I would be OK with just mineral oil, but not around a sink. Water has the amazing ability to get into the slightest gap and swell the wood, leaving a wider gap for even more water.

Thanks so much, I appreciate your input!

Well, I think I'm convinced not to go the mineral oil route with our planned undermount sink in our planned butcher block island.
Part of my reason for wanting butcher block was to "break up" the colors and textures in my kitchen. Too much "matchy matchy," I think (though it isn't built yet). And also to tone done the extremely modern ambience of the Ikea cabinets we will be having installed.

Does it sound too crazy to use porcelain tile for an island countertop AND what about using the "porcelain wood plank" that is increasingly popular? Would look like wood. I don't really care about fooling anyway, but I do like the LOOK of butcherblock.

And I still would like an undermount sink but think that these days, there are techniques of doing an undermount sink with tile. Gotta do more investigating. Glad I found this thread. I think it has made up my mind to NOT go the direction I was going in. That is important.

I would check out the Ikea reviews. I remember seeing some where there were some problems.

We have Ikea oak bb on island, not around sink though. It's finished with Waterlox, and has held up great when drinks condensate on the island.