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I'm finishing a hard maple joiner's mallet with boiled linseed oil (BLO). The instructions on the container say to cut it with 2 parts mineral spirits to 1 part BLO. Is this strictly necessary? From what I've read, thinning the BLO will let the finish penetrate the wood deeper, but I'm not sure whether that's necessary for a mallet which will get beat up either way.

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No you don't need to thin BLO, or any other drying oil, to apply it.

From what I've read, thinning the BLO will let the finish penetrate the wood deeper

That's the theory! But it's one of the most persistent myths in finishing. In reality it doesn't appear to make any real difference (except possibly in end grain).

People have run tests that have shown that it increases penetration and other tests have been run that show zero difference* and it's important to note that where deeper penetration was measured the improvement was minuscule. That kind of thing might be important to a luthier working in thin sections of wood, but for a bulk item like a piece of furniture or a mallet it's hard to imagine that an extra 0.1mm is going to make any appreciable difference!

From trying it both ways I can say that, empirically, the wood looks exactly the same, there's no difference in durability that I can notice and the surface feels no different, as long as you go on to apply enough coats in total — there is a massive difference in all three of these if you wipe on three dilute coats of BLO versus three undiluted coats.

Another option you might like to experiment with is to gently heat the BLO to reduce its viscosity (best done in a double boiler or some similar arrangement, not over direct heat). This supposedly increases penetration too. If you try this it's quite evident it does reduce viscosity, the oil is visibly more runny, however once applied to the wood it again appears to make little or no difference in how deeply the oil is absorbed. I'm actually a fan of doing this, especially during cold weather, but only as a means to easing application. I don't delude myself that the oil is going in any deeper.

*What these tests may inadvertently have proven is that the type of wood is a key factor in relative oil penetration, but that shouldn't have been any surprise anyway.

  • Small nitpick on "drying" oils. Linseed oil is not drying. It does not lose water or another solvent. Instead, it undergoes an irreversible chemical reaction with oxygen (that's why air is important, and why several thin coats are better, and why one must wait for the previous layer to cure completely before applying another, etc etc). If it was merely drying, you could simply wash it off (you can't). – Damon Jul 10 '16 at 11:42
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    @Damon, re. "drying oil" I didn't coin the term, that's what they're called in English (see Wikipedia entry for confirmation). And *cough* as you might expect I'm aware of the mechanism by which oils of this type cure ;-) – Graphus Jul 10 '16 at 16:24
  • @Graphus 0.1 mm??? I'd call that a very large amount of penetration and would be very happy if i could get a finish to penetrate that far into long grain. Do you have any sources you can point me to? – aaron Jul 11 '16 at 10:35
  • @aaron, penetrating finishes can go about that deep at the bottom of a pore in an open-grained wood, generally it's much less for close-grained woods. But even if you could increase penetration to 0.1mm uniformly it wouldn't really have any benefit because "[in terms of protection].the amount that a finish penetrates is of no significance." (Bob Flexner). You can get substances to more deeply penetrate even face grain using pressure and/or vacuum, as in stabilisation, but this is generally only viable on small pieces and it requires a liquid of low viscosity. – Graphus Jul 11 '16 at 13:52
  • @Graphus ah ok, i always forget about open grained wood, i tend not to use them. makes sense. right now i'm actually experimenting with using a very light coat of unthinned poly as a way to prevent blotching on cherry - based on the idea that I want to minimize any penetration into the uneven grain. – aaron Jul 11 '16 at 15:24
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Graphus gave an excellent answer. Once, having asked the same question to a guy very experienced in wood finishing, he explained to me that thinning the oil doesn't make oil molecules any smaller—it only spreads them further apart. Molecule size determines how deep it will penetrate and, like Graphus pointed out, an impression that thinned oil "should" penetrate deeper is just that—an impression. You only end up with unevenly-distributed oil, which is why it requires several more coats to reach the same effect as with pure oil.

note of advice: BLO contains added trash that makes it cure faster, it's not a pure oil. It also yellows and darkens over time. I always use Hope's pure tung oil and I've had much better results.

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