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Finishes: Claims, Myths, and Questions (Read 135 times)
 
Don Stephan
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Finishes: Claims, Myths, and Questions
Jul 8th, 2019 at 7:23pm
 
If memory serves, for a long time finishes generally have been classified as being either penetrating or film forming, with no suggestion of a middle ground.

Raw wood is not a solid substance like plastic or metal – there are air spaces between the cell walls and within them.  The larger cells in red oak for example are visibly hollow to the naked eye.  But it is never explained how the size of molecules in the various finishes compares to the diameter of the spaces between and within wood cells – can the molecules of finish actually fit within these spaces?  If they are small enough, do they actually enter these spaces?

Sometimes the rationale that a finish is penetrating is the statement that when a finish is applied to wood the surface quickly becomes dry,“ implying the finish molecules penetrate into the wood.”

The three most common solvents used to thin finishes may be mineral spirits, denatured alcohol, and water.  All three are rapidly absorbed into raw wood.  So when a finish containing one of these solvents is applied and the surface “becomes dry,” are molecules of finish being drawn into the spaces in the wood with the solvent, or just the solvent?

A three pound cut of shellac is relatively thick, and many might label it a film forming finish.  A one pound cut of the same shellac is much less viscous, and often is suggested as a “sealer” to reduce or eliminate stain blotching on cherry after light sanding, as it does not completely block absorption of stain into the wood.  The implication would seem to be that this “wash coat” or one pound cut shellac leaves some molecules of shellac within the spaces of the wood and some molecules on the surface.  Does this mean shellac can be both a film forming and a penetrating finish, and if so what is the value of the classification?

Can the same questions be asked of varnish straight from the can versus the same varnish thinned 50% with its solvent?

If a thinned version of a finish “penetrates” the wood while a thicker version forms a film on top, what would be the benefit of applying several coats of the thinned finish to fill the spaces between and within the wood cells, and then brushing a thicker version of the same finish to create a film on top of the filled wood?  I’ve never seen this suggested or recommended, but at first glance it would seem to offer a different result than one or the other.

Does a sprayed lacquer sanding sealer penetrate the wood the same as a brushed one pound cut of shellac?  Both are sometimes simply called “sealers” but do they interact with raw wood identically?

If memory serves, boiled linseed oil and pure tung oil are usually or always classified as penetrating finishes.  But if enough coats are applied, allowing adequate time for each coat to cure, boiled linseed oil and pure tung oil eventually build a layer on top of the wood.  Is this film conceptually different from the film resulting from straight-from-the-can varnish?

With time, woodworkers come to know what results to expect from finishes they use repeatedly, perhaps without an accurate understanding of the answers to the above questions.  But for less experienced woodworkers, marketing statements and casual conversation can be very confusing and even misleading.  I might be wrong, but it would seem to me that well defined and carefully used terms and definitions, and understanding of what is actually happening when finish meets wood, would be tremendously beneficial to all.

(Also posted on WoodCentral Messages forum)
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Ed Weber
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Re: Finishes: Claims, Myths, and Questions
Reply #1 - Jul 9th, 2019 at 12:07pm
 
I'll try to answer what I know
There are really only two types of finishes, penetrating and film forming.
We usually refer to them as oils and varnish.
There is some cross-over
Some penetrating finishes can be built up with many coats to achieve a thin film, Tung oil is an example.
Film building finishes, what we usually refer to as top coats can be thinned but they don't penetrate in the same way oils do. A thinned film building finish flows more easily into smaller areas of the grain and large pores and then cures to a film that can built upon.

Certainly not a scientific answer but I use the analogy of coffee.
When you brew coffee the stained water is what you drink but the solids remain because the filter won't let it through. This is similar to using a top-coat that is being used as a penetrating finish.
You can thin a top coat and to a small degree, some may penetrate a slight amount but the size and composition of the solids in the mixture will do a few things.
One is that it will penetrate slightly but will be weak and thin requiring several more similar coats to build.
Two, it can act like the coffee analogy above. The stained solvent will penetrate but very little of the solids will be able to be absorbed due to their size and composition.

This brings us to the wash-coat scenario. What you are doing is trying to partially clog the larger pores so that they are similar to the rest of the wood. Applying a thinned coat that penetrates slightly into the larger pores limits how much they will absorb. After a light sanding the larger pore areas like end-grain and soft early growth areas will have a small amount of the shellac remaining.  This can even out the absorption of other finishes and reduce the potential blotching, although there are no guarantees.
This is the art/science portion of finishing

The very popular 1:1:1 finishes that are comprised of one part oil, one part varnish and one part solvent are nothing more than home made danish oil.
These are not magic recipes or secret finishes and retail danish oil is nowhere near a 1:1:1 ratio.
When you use a danish oil, the directions want you to flood the surface of the piece until it can't absorb any more product, why is this?
It's because you only get one shot. Once the thinned oil and varnish in the mixture starts to cure, the wood won't be absorbing any additional finish, so it's imperative that you get as much finish absorbed into the wood as it can take. I have seen some turners that entirely submerge their pieces in a large vessel of danish oil overnight to achieve this. Any additional coats will be building on the surface, mainly utilizing the varnish portion of the mixture.
This is why retail danish oils and wiping varnishes don't have much oil in them, it's not necessary. All you want the oil to do is enhance the grain, it doesn't take much at all and is only beneficial during the initial coal.  The solvent helps with application of the oil (thinning the viscosity to help it flow) but there is debate over whether it actually helps the oil absorb deeper into the wood. Once the solvent has evaporated, what you have left is 100% oil (whatever you started with) that will start to polymerize, just less of it since it was thinned. After the first coat, the benefits of the oil become less and less.
There is always a compromise when using an all in one finish. You inevitably sacrifice something in exchange for ease of use.

Don Stephan wrote on Jul 8th, 2019 at 7:23pm:
marketing statements and casual conversation can be very confusing and even misleading.  I might be wrong, but it would seem to me that well defined and carefully used terms and definitions, and understanding of what is actually happening when finish meets wood, would be tremendously beneficial to all.



I can see about making a list in the finishing forum and leave it as a sticky.



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Don Stephan
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Re: Finishes: Claims, Myths, and Questions
Reply #2 - Jul 9th, 2019 at 7:30pm
 
Regarding your comment that commercial danish oils don't have much oil in them, I think I read in an article by one of the well known finishers that write a lot, the suggestion was that a danish oil mix of 1/9 BLO, 4/9 varnish, and 4/9 mineral spirits would yield the same look as equal parts of the three, and be a harder finish due to the smaller BLO content.
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Ed Weber
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Re: Finishes: Claims, Myths, and Questions
Reply #3 - Jul 9th, 2019 at 8:11pm
 
That would be a better ratio for making your own.
Retail versions are closer to 1/9 oil 2/9 varnish and 2/3 solvent.
This is an educated guess, the MSDS sheets don't include much information to go on these days.
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« Last Edit: Jul 9th, 2019 at 8:12pm by Ed Weber »  
 
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Mike Nathal
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Re: Finishes: Claims, Myths, and Questions
Reply #4 - Jul 9th, 2019 at 8:21pm
 
I enjoyed reading both of the above posts.  I can add only a little:  1.  I believe the size of the finish molecules (eg, varnish molecules) are still quite tiny compared to the pores in the wood.  Perhaps by a factor of 1000 or more.  I think the viscosity of the finish determines the depth of penetration. 
2.  Bob Flexner's book "Understanding Wood Finishing" has some good schematics of how different types of finishes penetrate and fill up the pores.
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Ed Weber
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Re: Finishes: Claims, Myths, and Questions
Reply #5 - Jul 10th, 2019 at 8:59am
 
Mike you're correct but the size of the molecule compared to the size of the wood pores is only one factor in how well or how far/deep the finish penetrates the wood. Just because "the marble is smaller than the hole" doesn't mean it will penetrate as deep as many think. If this were the case, finishes would go in one end of the pores and fill until it came out the other end, this doesn't happen. (there are exceptions)

Without even considering the wood species, grain pattern, oil content, water content, relative humidity, and other factors. There are things like the capillary effect, adhesion, cohesion and the makeup of the finish, oil-based, water-based, etc. All these things are variables in how and why a finish penetrates or "sticks' to the wood.

Just an FYI, Multimedia File Viewing and Clickable Links are available for Registered Members only!!  You need to Login or Registeris considered a top-coat according to their own info.
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