Carob burl bowl with UV
resistant "enhanced" oil finish
UV Resistant Finishes - Overview
As woodturners, we're always concerned with the effects of ultraviolet (UV) light degradation and color loss in our projects. Many woodturners (myself included), have experimented with blending different finishes in an attempt to lessen the damaging effects of UV light on exotic timbers, as well as local timbers.
Through the years, I've found out that making your own UV resistant finishes is no easy task! The UV stabilizers are very expensive and hard to obtain in small quantities. Further complicating the matter is the fact that UV degradation and protection is a very complex subject, requiring extensive research and testing to develop useful protocols or finishes.
While I'm not a chemist, I have done quite a bit of research and testing through the years on projects in my own woodturning studio. I've achieved success with some protocols and failed dismally with others. If you want to try you hand at making your own UV resistant finish blends, here a bit of what I've learned to date:
UV Resistant Finishes - Challenges
The UV stabilizers used in manufactured finishing products work very well in most situations. They are backed up by fully equipped labs and chemists that specialize in the effects of finishes that are exposed to the elements. It's not a simple process for a the average person to make a UV resistant finish from scratch.
There is a ton of chemistry involved in determining the right type and amount of UV inhibitor to use. Also, to test the effectiveness of a UV resistant finish requires the use of an accelerated UV test machine, or close observation of subject pieces over a period of many years.
To complicate matters further, various types of UV inhibitors are available. If you're trying to reduce UV degradation on your turned pieces, it's generally easier to utilize an existing manufactured finish, or a blend of finishes that incorporate a good UV stabilizer package (like a high quality marine varnish), than to try to formulate your own.
One problem with using a marine varnish straight from the can is that it leaves a very thick coating on the surface of the wood. However, you can easily thin a marine varnish for better penetration, or blend it with another oil finish to increase the base finishes UV protection level.
Marine varnishes with high amounts of UV inhibitors are expensive because the UV inhibitors used are expensive. If you've ever owned a wooden boat before you know this all too well. You also know that even with the best protection, you still have to reapply the finish periodically because of the continuing effects of sun, wind and rain on the finish.
Luckily, refinishing is not necessary with woodturnings, since our projects are usually stored inside. However, they may still be exposed to sunlight if placed near a sunny window and over time, this exposure can cause vibrant colors to fade, or disappear entirely.
The easiest way for you to boost the UV protection in one of your oil finishes is to find a compatible manufactured finish that contains a high amount of UV inhibitors and blend some of it into your desired finish. This may not be the only option, but short of getting a degree in chemistry, it's probably the best option and the least expensive solution for most woodturners.
Even if the UV finish is expensive, you will be blending it into another finish so the overall cost per project should still be affordable. Here is a bit of information to help you understand more about UV resistant finishes, the types of inhibitors used and how they work to protect against the damaging effects of UV light.
UV Resistant Finishes - Primary Stabilization Methods
Two primary methods have been adopted to stabilize light/UV light:
1.) Competitive UV absorption by UV absorbers in the 290-350 nm wavelength range
2.) Trapping of the radicals formed during polymer degradation by radical scavengers using Hindered Amine Light Stabilizers (HALS).
Two systems you should consider when making your own home brewed finish are chemical and pigmented.
Chemical: Ciba Speciality Chemicals has a HALS stabilizer, which is a liquid amine stabilizer. It consists of an almost pure mixture of Bis (1,2,2,6,6-penatamethyl-4 -piperidinyl) sebacate and Methyl (1,2,2,6,6-penatamethyl-4 -piperidinyl) sebacate. It's used in automotive coatings, wood stains and industrial coatings and is a clear chemical liquid.
Pigmented: Certain pigments will prevent the UV light from attacking both the coating system and the substrate. The best type is the old Zinc Oxide and the newer Titanium Oxide, which forms a complete block out. Other pigments of course will provide similar protection, but will obscure the grain. The Transparent Iron Oxides and Titanium Oxides have a very fine particle size and are therefore, transparent.
Transparent Iron Oxides are less expensive, but they are colored. However, the color can be used to enhance the surface of the timber. Since they are nearly clear, they do not obscure the grain. All UV systems protect both the coating and the surface that the coating is applied to. However, one also must consider the properties of the coating itself, is it a varnish film, or a penetrating finish?
Caveats with UV Resistant Finishes
Thick finishes with UV protection (like marine varnish), do not really penetrate the wood surface deeply. Therefore, a breakdown of the coating allows deterioration of the wood surface by allowing moisture to get under the coating. This delaminates the remainder of the finish.
A penetrating oil finish has the advantage of soaking into the wood and does not form a skin that can lift. A varnish finish has certain advantages however, because the film forming thickness penetrating oil finishes provide is substantially less than multiple coats of varnish.
The Oxides being inert do not lose their effectiveness over time (vs. the chemicals), but if the surface coating deteriorates, the UV factor is decreased. So, it is a bit of a trade off, if the coating breaks down the UV light will get through and attack the substrate.
Penetrating type finishes offer less breakdown of the surface coating, but do not provide as thick a surface film layer. As woodturners however, thick layers of a surface finish are generally less desirable than finishes that provide a "closer to the wood" feeling, like a penetrating oil.
I have experimented quite a bit with blended finishes in an attempt to strike a balance between the better overall coverage of the chemical systems and the superior long term performance of the pigmented systems. I have achieved excellent results with several protocols, but I think I could spend the rest of my life looking for the perfect UV resistant finish for woodturners and never come close to that goal.
For most woodturners looking to limit UV degradation in project pieces, using a manufactured finish with a high amount of UV stabilizer (like a Marine Varnish) is the most logical way to add extra protection to one of your oil finishes. Thinning a Marine Varnish will reduce its overall effectiveness.
However, unless you want a heavy film finish on your project, you will probably want to thin the varnish to make it more like a wiping oil finish anyway. Even with a reduced level of protection afforded by the thinned Marine Varnish, you will still have more protection than using a regular wiping oil finish that contains little, if any UV protection at all.
English Brown Oak burl bowl
with UV resistant "enhanced" oil finish
Safety Note: Always follow all manufacturers safety instructions before working with your lathe, or any of the tools or products you may use. If you are unsure about any operation, obtain competent professional instruction before proceeding. Use and wear all necessary safety devices during turning and observe safe woodturning practices to prevent accident or injury.
Steven D. Russell is a professional studio woodturner, teacher and writer. He has written numerous articles for international woodturning magazines, which have been published in more than 78 countries around the world. Steve has demonstrated in numerous cities across the United States. His studio, Eurowood Werks, specializes in bowls, platters and hollow forms with unique visual and tactile treatments.
Steve is also the current and founding President of the Lone Star Woodturners Association, Inc; an AAW member chapter. The LSWA is a 501(c)3 non-profit educational organization dedicated to teaching and demonstrating the art and craft of woodturning.
Steve is also a featured writer for the Guild of Master Craftsman's "Woodturning" magazine, published in London England. Woodturning magazine is the world's leading magazine for woodturners. Look for his articles covering technical topics, or project based articles in an upcoming issue.