oil penetration test

[Stan Calow]

I had some spare time waiting for the glue to dry, so I though I'd do a test out of curiosity. The goal was to see how much penetration there would be, since this is supposedly one of the advantages of oil finishes. I took some cutoffs of, cherry, walnut, white oak, mahogany and maple, and sanded one side to 150, the parallel side to 400. Then applied several finishes - pure tung oil, BLO, thinned BLO, danish oil, poly, thinned poly, shellac, & Waterlox.
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After a week, I cut cross-sections to see how they penetrated. As expected, none of the film finishes showed any penetration (so I dont show those). But I was amazed to see no noticeable (to my eye) penetration on any of the oil treated cutoffs - none. I was expecting about 1/8" or so.
I am only showing the TO, BLO and thinned BLO. The side grain was not part of the test.


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Not satisfied, I took cutoffs of mahogany and walnut, without any sanding (i.e., planer finish) and put heavy, un-wiped coats of BLO, and another set I let soak in a cup of BlO for a few hours. I saw no penetration on either mahogany samples. The walnut seemed to show very minor penetration (1/64" or so) on both these samples.

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Maybe I'm way off, and am wondering if anyone else has seen the same thing, or maybe a magazine has looked at this. Its certainly contrary to my expectation. I was really hoping to see if thinning BLO would have an impact or not.

Your thoughts welcome.

[Prashun Patel]

I've done this test before and have reached the same conclusion: there's surprisingly LITTLE penetration with any kind of finish, even stains and dyes and glues and epoxies.

The results differ when testing on end grain with certain types of wood; in that case woods like red oak will drink up liquids like a frat boy.

There is at least one exception to this: dyes solubilized in acetone will penetrate a little deeper. However, this is when you soak it. Typically, nobody would soak a wood in a solvent dye; it'd be sprayed on. If you try to spray an acetone dye, the irony is that you'll get even less penetration than with water because the majority of the solvent flashes off through the air; so you end up largely dry misting the piece. This can be a great tool when toning, but i am in no way suggesting the avg person try this; acetone is dangerous on several levels.

We make a big deal about glue penetrating into the wood if you wipe it with a cloth while it's wet. I find this only to be an issue in corners where your ability to sand even a little bit into the corners is compromised. But on any kind of open seam, there's 0 harm in rubbing all the glue around and off. It's short work to sand or plane even rubbed in glue after the fact.

[John TenEyck]

Same results here. I did some tests to see how far dye soaks in. My observations with magnifying lenses should just a few thousandths of an inch. It's all a surface phenomena. And that showed me why it's so easy to cut through a stain or dyed finish when sanding between coats.

[Stan Calow]

What a relief that two respectable sources provide confirmation! I had always heard that oil was an "in the wood" finish, which would heal itself from shallow scratches, and that many people thin oils to increase penetration. Now I am inferring that additional coats of oil don't increase penetration either, but provide a greater surface buildup. Therefore multiple coats of oil create a softer film surface, more susceptible to scratches or rubbing off. Maybe thats why the mix of oil and varnish (like danish oil) is popular?