dudeman555

Hi all, I made 2 Bathroom mirror frames; sized approximately 3x4 ft.
They are solid cherry wood. I need advice on the use of tung or
linseed oil as a finish for this particular application. Will the moisture
be an issue?

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firefighteremt153

If they are going to be in a bathroom then I would think that moisture would indeed be an issue. If they were mine, I would use the linseed oil followed by some poly. The linseed oil will enhance the woods figure and grain while the poly will give it the protection it needs. Thats what I would do.

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jerry

While no common finish will stop water vapor any good varnish will slow it down.

Jerry

Wood4Fun

i agree with previous posts... either oil will likely be nice and will bring the wood detail out, but neither will provide the vapor protection like a nice coat of varnish over either.

Dejure

The advantage of the oil application before the poly is you can get a penetrating coat, which could take the seal coat to new levels, so to speak. You can use tung oil or linseed oil. The latter is cheaper in the pre polymerized category of things. Or you can use raw tung oil and let it dry a couple days. Thinned, it should be dry in a day or two.

Hit the back side too, just like you would exterior siding, to stop moisture transfer behind the scenes.

rrich

If it were me, I would use a commercially prepared curing oil. Allow the oil to cure for a week or so. (When you can't smell it any longer it is cured.) Then a few of coats of a polyurethane. If the oil is fully cured, a water based polyurethane would work nicely. If you want a satin or semi gloss finish, two coats of glossy polyurethane and a top coat of either semi or satin.

Commercially prepared curing oil is Watco or Minwax Antique Oil Finish. (My preference is the latter.)

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Rich (Tilting right)
Huntington Beach, California
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Julian the woodnut

I say just lay on the poly with no oil beforehand. If you use an oil based poly, it will have linseed oil in it as a solvent anyways, so there will be no need to use linseed oil first. If you want to seal the grain first use dewaxed shellac first.

Dejure

The commercially prepared oils from Watco and Minwax are wiping varnishes. These are really nothing more than thinned versions of varnish (e.g., resin and oil). You could get the same effect, at a fraction of the price, thinning straight product.

That said, that too is an option - thin about twenty-five percent for your initial coat(s), to get penetration. That is the key to why many of us promote oil and an initial application before the final. It's the difference between a mere surface coat and a penetrating finish.

Decades ago (literally) I did a burl table top. I loved Plastic Oil, put out by Varithane, but wanted something less expensive, since I was not sparing in its use (keep in mind, most commercial users don't use more product only because it cost in materials and labor). I just thinned Varithane about thirty percent (didn't know about adding oils and such back then). I kept adding thinned product to the six inch slab as the previous coat was absorbed. At the end of the day, the finish had penetrated entirely through the slab. Once the solvent evaporated, I was left with what was, essentially, a piece of plastic wood. Thirty years later, even after sitting in front of a fire place all that time, the piece has not shrunk and cracked from moisture loss.

A big advantage of an oil base for your finish is, even if your surface coat fails, the penetrating finish will protect.

Failure is not just cracks allowing moisture in, but also cracks allowing moisture to escape. One causes expansion and other contraction of the wood. If your finish doesn't have some flexibility, it will crack.

With this in mind, you could experiment. For example, if it mixes, it's fair game. That is, if it's an oil based finish, you can add oil, paint thinner, turpentine, or naphtha, or polyurethane.

Adding more oil to polyurethane makes it more flexible and tolerant to expansion and contraction of the material being protected.

If you "soften" your initial finish applications, you can still apply a straight top coat.

Finish alternatives are many and varied, but much of the commercial stuff promoted as top shelf stuff is not anything remarkable. Often, equally good product can be made at home in seconds more than the time it takes to open the lids of a few things.