Mixing Finishes?

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Forum topic by CaseyJones posted 01-31-2013 04:30 PM 769 views 0 times favorited 6 replies Add to Favorites Watch
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7 posts in 2328 days

01-31-2013 04:30 PM

I’m about to finish a large walnut slab (8’ x 30” x 1.75”) It will be mounted into the wall as a floating shelf/counter, with metal support brackets. I’ve had good results finishing walnut like this with just oils, as well as a wiping varnish like Waterlox. In this situation, the slab needs to be mounted in the wall first, at which point the edges and part of the bottom will be covered in a natural clay plaster (the wall finish) where it intersects the wall. I’d like to have the ability to finish the top with a hand rubbed oil finish, but seal the bottom and back sides that will be buried in the wall with a less permeable finish, like a varnish, not necessarily an epoxy or something very heavy duty.

I realize this is not conventional, because you want the slab to respond the same way to seasonal wood movement due to moisture, but the underside of the slab will not have air contact while the top side will, so moisture absorption will be different anyway. I also want to protect the parts of the slab that will come into contact with the wet plaster.

My intuition tells me this is a bad idea, but I’d love to hear if anyone has firsthand experience.

6 replies so far

View Grandpa's profile


3259 posts in 2701 days

#1 posted 01-31-2013 04:39 PM

I agree with your intuition. Lots of moisture on the bottom of a board only 1 3/4 thick. I would seal that before I put anything on it. I could be wrong but my gut tells me to not wet only one side of raw wood.

View RussellAP's profile


3104 posts in 2312 days

#2 posted 01-31-2013 04:54 PM

A simple solution is to do the plaster and set the board in it, then remove it and let the plaster dry. You can cut some air grooves in it and even stand the board off it for a bit of circulation under the board. I know that board will warp if you put one side in plaster.

-- A positive attitude will take you much further than positive thinking ever will.

View Cosmicsniper's profile


2202 posts in 3184 days

#3 posted 01-31-2013 05:11 PM

How much of the board is going into the wall? I think you might be over thinking it a bit, unless this is a really thick wall. If you finish everything with oil and then varnish the part in the wall, I see nothing to worry about.

Is your worry actual liquid moisture or just humidity. That makes a difference.

I guess an illustration would be nice.

-- jay,

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11780 posts in 2405 days

#4 posted 02-01-2013 06:24 PM

Oil and varnish have very different moisture permeability, the board will warp. I like Russell’s idea because chances are that even if you finish both sides in varnish it will warp quickly if set in wet plaster.

-- Rick M,

View RussellAP's profile


3104 posts in 2312 days

#5 posted 02-01-2013 06:42 PM

If that’s an eight foot walnut slab under 2” thick, I bet it’s already twisted and or warped, but it’s soft enough to lay back down with just a slight tweak depending on the figure in the board. I’ve never seen walnut go 8 feet with out some major event in the wood though. Finishing it on only one side will warp it, finishing it on all sides will likely put a slight twist in it. This is why I work with 2” walnut at minimum for large runs.

I’m still unclear about the final installation and a pic or drawing would help.

If the walnut has to be installed in the wall first then plaster applied over it, you could use some clear plastic wrap like Sarann Wrap between the wood and the plaster and simply cut it off when the plaster dries.

-- A positive attitude will take you much further than positive thinking ever will.

View CaseyJones's profile


7 posts in 2328 days

#6 posted 02-01-2013 08:51 PM

Thanks for the feedback, I’ll try to post a sketch tonight, but basically the idea is that there is a 16” thick curved masonry wall, the slab is mounted to brackets for support, but a portion of it will be buried in the finished wall. It will be impossible to do the plastering before the slab is set in place do the details of the project. The slab has definitely already twisted a bit, but I am able to move it back to where I want it with a combination of steel angle iron supports where it will be attached, and hidden blocking in the wall.

At this point I’m leaning towards prefinishing the entire slab with Waterlox, which i’ve had good luck with before, and wrapping it in plastic before installation. I’ll get a sketch posted soon though since this is by no means a typical installation and is tough to visualize.

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