Why is it considered good for a finish to penetrate?

[Tetsuaiga]

I notice this in marketing but also on various posts or people talking on YouTube.

But what use is a finish inside the wood, surely the only place it does anything useful is at the surface or above it?

I suppose a hard finish like shellac or varnish that penetrated might be good in hardening the wood under the surface but I mostly hear penetrating mentioned with oils which I don't feel really give any protection in terms of strength/hardness.

Does anyone have any thoughts?

 

[sunnybob]

oils are more for colouring , so the deeper the penetration the longer the colour lasts

 

[Just4Fun]

oils are more for colouring , so the deeper the penetration the longer the colour lasts

Is that really true? My kitchen worktops are oiled and this is certainly not for colour. It is so that any spillages do not seep into the wood. So it is for protection but against liquids rather than against knocks. Incidentally the oil has to be renewed every few months or this protection is lost and this is clearly visible.

I am not convinced that oil really soaks in very far anyway. I replaced an oak worktop in my kitchen and have been re-cycling the old worktop. This has involved cutting & re-sawing it into smaller pieces and planing them. Sure, the surface is definitely oiled (which I cleaned up with a scraper) but not far below the surface the wood is just like new timber with no sign of any oil having reached it. I initially thought I would have to stick to an oil finish because of the old oil but now I think I can treat the recycled wood any way I like because the oil never penetrated very far.

 

[ED65]

Deep penetration has long been a catchword phrase or a marketing tool of the finishing industry. Ads that go back to at least the 40s emphasise it and it could go back further.

Not all finishes are penetrating finishes of course, but for those that are it makes sense to emphasise how deeply they penetrate the surface... but most of the claims in this area are porkies, or at least misleading. If a given product is guaranteed to penetrate very deeply they may only be talking about a tenth of a mil difference (if that) on long grain! Hardy what the man in the street would think of as deep.

Wood is just not that absorbent. Even with punky wood as sometimes found in spalted pieces the penetration of very dilute products specifically geared to penetrate deeply can be startlingly shallow.

But even given that the difference between 'deep penetration', as you get with oil, and shallow penetration, as you'd get from a heavier mix of shellac, is incredibly slight it is still visually significant. This is why some still go to the trouble of oiling wood prior to French polishing it, because the improvement in the look of the wood is absolutely worth the extra step.

 

[ED65]

oils are more for colouring , so the deeper the penetration the longer the colour lasts

Is that really true? My kitchen worktops are oiled and this is certainly not for colour.

Different type of oiling for a different purpose.

I am not convinced that oil really soaks in very far anyway.

Yup. On many woods that have been oiled if you look at a section across the long grain you'd need a powerful magnifier or a microscope to see the penetration.

End grain however is a different story. All woods have absorbent end grain but in some there's nearly no limit to how far the grain will transport a liquid, q.v. the old practice of filling the mouth of a wooden plane with oil and waiting until it is weeping out the toe and heel :shock:

 

[Tasky]

Different type of oiling for a different purpose.
All woods have absorbent end grain but in some there's nearly no limit to how far the grain will transport a liquid,

I agree!
For about the past six or eight thousand years, weapon-makers have stood the staves of wooden polearms in buckets of linseed oil. In the case of an oak quarterstaff, instead of just a big stick you'd end up with something that hit like a concrete baseball bat!
I promise you that the oil can penetrate very deeply if you do it right...

 

[Tetsuaiga]

^That's quite a niche area.

Would oil ever actually cure if it were that deep in the wood?

Interesting to hear about bothering to oil before French polishing for the appearance difference.

 

[Tasky]

^That's quite a niche area.
Would oil ever actually cure if it were that deep in the wood?

Niche-ish, perhaps. But it does illustrate the wicking properties of the grain on long bits of wood.
Not sure about whether it actually cured or not. I wasn't into woodworking back when we did this... we just left it until it was fairly saturated, and then stood it on paper until it was touch dry again. Certainly felt fantastic!