Here is an article that may answer your questons: https://abeautifulmess.com/2014/09/diy-butcher-block-counters-with-undermount-sink.html

If it were me, and I was determined to install butcher block, I'd mount the sink in a more impervious surface, like a piece of granite, so that there was at least 6" of clearance on either side before I began the butcher block. Depending on your counter layout, this could look great or odd, depending on how it was done. The one positive: I suppose it's better to use walnut than the standard maple as the water damage and staining will be less apparent with a darker wood. I will also point out that an undermount sink will allow the edges of the butcher block to be exposed to water, and end-grain is where the wood requires the most sealing and where it will most readily absorb water. With an overmount sink, you would only have to worry about maintaining the top surface.

Wooden counters with sinks Installed are very common in the UK. I can't advise you how to proceed, but I am sure there is info out there for you. Plus, I am sure your counter will be absolutely fine with a little care. If it were me, I would be maniacal about sealing the wood well.

Love the photo above, shared by wilson853. Would never have guessed that could look so fabulous! Seems like a great solution! What do you think, csummersall?

Let the exposed end grain sink edge suck up several coats of thin epoxy please.

https://www.odiesoil.com/. I just found this. I’ve never used it but worth looking into.

Beautiful, classic option, IMHO. I have friends with undermount in wood, and I have used two or three in UK rental homes. Seal it regularly, and be a tiny bit obsessive about wiping round the rim and all will be well. Please come back with before & after shots!

$200 for walnut counters for the whole kitchen??!!! Wow. I would have jumped on it, too. We just did maple counters at our lake house and I felt we got a good deal at about $700.

We did an overmount sink, but still looked into finishes pretty carefully. I found that Waterlox and Good Stuff for Wood were the most/best recommended products here. Neither of these are available in my city. We ended up using Watco's Butcher Block finish. Our sink was just installed a few months ago, so I can't say how it is working, but the finish is very hard and water beads on it - even when left for awhile.

We have had wormy maple on our island counter in our main residence for 5 years, but there is no sink. We tried a variety of finishes on it including a mix of oil and wax from Boos. This had to be re-applied often (we did it monthly, but it could have easily been done weekly). It kept it from drying out, but was not protection from water or staining. We also tried Osmos. This gave it a little bit of protection, but not much and also needed to be re-applied about every 6 months to a year. We currently have Watco on it and it has held up well. We did 5 coats initially and after a couple years, I feel it needs another coat (but this is easily done).

If you prefer a poly finish, we used Old Master's wipe on poly on our old oak farmhouse kitchen table. It got lots of use as it was in the middle of our kitchen and this held up really well. I debated using this on our new maple counters, but felt that I wanted something that sunk in more rather than sitting on top like a poly.

I am not an expert, but the above has been my experience. Hope it helps. Would love to see pics when it's done.

Great to have the experiences and advice from daisychain01!!!