Protecting Wood and Coatings from Yellowing

      Here's an extended technical discussion of wood yellowing, finish yellowing, and the finish formula components that may or may not prevent either one. August 21, 2012

Question
Is a natural stain necessary in the basic natural clear coat lacquer finishing process? Someone told me it helps prevent the wood from yellowing, particularly if used on pine. I have a hard time believing this, and am not sure the extra labor and time is warranted.

Forum Responses
(Finishing Forum)
From contributor C:
I wonder if the reference to a clear stain might be a wash coat of shellac. I have used this under a varnish or polyurethane finish and it can help keep the final appearance lighter and less yellow. I'm not convinced that it would make any difference under a lacquer finish, as lacquer in my experience has no yellowing tendency.



From contributor D:
Some have said when they use BLO under the film coats, it makes the wood pop. I have never heard it stops UV damage. Most brands make a natural stain. It can be used as is or with another color to make stain of choice. I've never used it, then clear coated.


From contributor F:
A clear stain base will do nothing to stop the natural process of wood changing color. A heavily pigmented stain will slow it, but not stop it. If the final coating has a UV inhibitor, that's what will defend against the changing of color. I feel that using a natural stain base does nothing but waste product, labor and time. If the proper coatings are chosen, you can self-seal with the same product and be done in 2 coats on most wood species.


From contributor J:
I agree with contributor F.


From contributor M:
The trick to using stain to reduce or eliminate yellowing is to use a pigmented stain, not clear. The pigment should be the same color as the natural wood color. For instance, birch would be stained with a "birch" stain. This puts a pigment layer on top of the wood to block out UV light and inhibit yellowing. "Staining" with any true clear would do nothing of the sort. Wood yellows a lot, and finish yellows practically not at all.


From contributor Z:
Have to disagree. Clear finishes that contain nitrocellulose do yellow. Take some of your lacquer, put it on something white, then stick it in the sun for a few days. It will yellow.


From contributor F:
Contributor Z is on the money. Any finish with nitrocellulose will amber with time. The cellulose part of nitrocellulose is basically wood fibers. We all should know what wood fibers do when exposed to UV sources. But even if the coating doesn't amber, that still doesn't keep the wood from changing! I don't know what coatings you are using or what a salesman is telling you, but there are only limited numbers of clears on the market that will not amber. Examples: ML Campbell products - Krystal, Klearvar, MagnaKlear and Klearplast. MagnaKlear has UV inhibitors added that should help with stopping the wood from changing. Water white does not mean that a coating will not amber; it means that it has the appearance of water (being able to see through it) when viewed in the container.


From contributor B:
Coatings with UV inhibitors do not keep the substrate from changing color. The UV inhibitors only protect the coating and are sacrificial. UV radiation is still going to oxidize the substrate.


From contributor A:
Using stain for reducing yellowing of your furniture is a good idea. I have used it and it is really useful. In this, a pigmented stain is used, not a clear one, so that it does not create a bad appearance and should also be of the same color as the furniture.


From contributor W:
I have to disagree with contributor B on this entirely. I've been using UV clears or varnishes in the marine coatings industry almost all of my life, and I can assure you that any product with good UV inhibitors will protect your base or substrate.

Yachts are subjected to the worst UV exposure of all, especially horizontal surfaces, and when a good varnish does start to craze and yellow, it must be stripped. Almost all of the time, the wood underneath has its color. A UV filter or inhibitor is just that, regardless of what is underneath it.



From contributor H:
I'm assuming "natural stain" is a clear base used to add pigments or dyes to. No stain base or shellac (though shellac does have good UV resistance) or other readily available solvent based clear natural stain can stop wood from yellowing or greying or darkening.

Regarding nitrocellulose, it is most assuredly true that cellulose nitrate does yellow. That which is used in wood finishing is not made from wood fiber; it is manufactured from cotton linters (the small fibers attached to the cotton seed after ginning the long strand cotton balls). Cotton comes in many grades from white to yellow, but all will yellow, no matter how water-white they may look in the can or crystal clear beaker.

Water whiteness is a determination (or was originally, seldom used anymore) used by cellulose nitrate producers to classify its yellowness. The ASTM test was a standard at one time where a small measured amount of potassium dichromate was added to pure water (4.8 to 5.6 mg dissolved in 1 L of distilled water). This was originally used for water and aliphatic and aromatic solvents with the D-365 testing done for darker cellulose nitrates accordingly.

Most oils used in wood finishing also darken. Linseed oil is one of the worst offenders, but the darkening or yellowing of any wood oil is not the same as the oxidation of the natural coloration of the wood.

If one wanted to use oil for the purpose of bringing about a better RI (refraction index), I would recommend the turpines such as pure gum turpentine, with a tiny amount of b-17 or window clear colophony dissolved with it. Personally, I would not use any oils.

As to stain bases being useless for inhibiting lignin oxidation (yellowing), that is correct, but there are other aqueous based materials that can stop lignin oxidation very well. They have not been on the market for long enough to really know how effective they will be in the long run, but their use over the last several years is extremely promising. Manufacturers have been using them with great success.

As to the use of pigmented stains, it is true they will greatly help in protecting the wood substrate from lignin oxidation, but on the other hand, if one prefers the totally natural look of the species, then any color added is not acceptable. Also, many woods have more than one natural coloring in them - darks and lights, reds to greys and yellows all within the same veneer or solid lumber. This also holds true when dye staining wood. Both require a deft application of material to even come close to looking like a purely natural wood. This is what I do when faux wood finishes are needed (see pics below).

I think contributor M is being misunderstood. From what I understand, he's not saying the finishes don't yellow, but that to whatever degree they do, it is not what is causing the lignin oxidation to occur. That will happen with most any coating, be it acrylic or cellulose acetate or butyrate or as a CAB component.

Regarding brand name coatings, I can only say that if, for instance, Krystal is a water white alkyd amino type finish such as SW water white conversion varnish or Guardsman, be aware that they can and do yellow if exposed to direct sunlight for several months or less. You can do as another suggested - simply spray a coat or two of your preferred A/A varnish over white and leave it on your dash half covered with cardboard. Every month, view the covered portion and compare. Yes, I'm well aware they are for interior use, but they do yellow.

The only long time proven non-yellowing coating on the market is made with either vinyl or acrylic. CAB, a hybrid cellulose product, is cellulose that instead of nitration with both nitric and sulfuric acids is made with glacial acetic acid and also butyric acid (the butylization portion of the mix) to produce the cellulose acetate butyrate compound.

Cellulose acetate on its own, over a long period of time, yellows to a small degree, especially when subjected to interior light. Butyrate resins do not.

I have not kept up with coatings as of late, so I am ignorant of any brand new non-yellowing resins that may have been introduced.

It is difficult to find a solid answer. The newest nano types are by far the best, but that does not mean the older, larger particle types were not sufficient for the job.

A UV additive is meant mainly for the protection of the coating, much more so than the wood itself. Its purpose is to stop radicals from prematurely oxidizing the film and thus the need for removing and recoating. It was never meant to protect the wood from lignin oxidative problems, though at the interface substrate level (where wood meets coating), there is some evidence of this taking place, but not to such an extent as a true lignin light scavenger does. HALS (hindered amine light stabilizers) are what is needed to protect the upper surface of the wood from darkening or yellowing, etc.

The first pic shows a faux burled wood grain. Different colors were blended to achieve the affect. In the second pic, the top is wood treated with both a UV stabilizer and HALS, the middle is with UV alone, and the bottom is neither, just the alkyd varnish.


Click here for higher quality, full size image



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