Hi all, I have a furniture project that is finally nearing the finishing stage. It is a pair of large chairs (thrones, actually) that will be used outdoors, though mainly in tents, probably an average of once or twice a month. I’d really rather avoid the heavy look of spar varnish – these have carving and inlay, and a thick film finish will detract from the appearance.
For protection against dew, damp grass, and the occasional unexpected rainstorm, I’m considering a penetrating epoxy sealer (CPES Multiprime). What I am worried about is the possibility of the epoxy deteriorating over time from UV exposure. So I have two questions:
1. Is this really likely to be a problem, given that the penetrating epoxy is mostly in the wood, rather than on the surface?
2. Is there a UV blocking finish that might protect the epoxy, but does not build a thick plastic film?
Thanks for any advice you can offer!
“Everything should be made as simple as possible, but no simpler.” A. Einstein
Replies
Epoxy does not like UV.
Epoxy with a light coat of spar vernish does well. The epoxy protects the varnish from the wood movement and the varnish protects the epoxy from the sun.
Ceramithane has UV blockers, Its an interior topcoat. Made for kitchen cabinets.
For the usage you describe and occasional dampness you want to guard against, I think I would skip the epoxy and use a penetrating oil finish. That's assuming the chairs are stored indoors when they're not in use.
A penetrating oil finish would ordinarily be my first choice for these - if they were to be used/stored indoors. Unfortunately they will be used outdoors and stored in a trailer. The polymerized tung oil finish used on a pair of thrones ten years ago was not up to the challenge... hence my search for something more robust."Everything should be made as simple as possible, but no simpler." A. Einstein
http://www.albionworks.net
There are two types of clear exterior finishes - film finishes and penetrating finishes. Film-forming finishes, like spar varnish, bring out the grain and figure of the wood and protect it from wear and tear. They are made with water resistant oils and/or resins; preservatives to block mildew, moss, and mold; and a mixture of ultraviolet absorbing and blocking chemicals for the most difficult task - reducing the damage caused by the suns' UV. UV energy damages the lignin in the wood; the "glue" that holds the wood fibers together. Without protection, UV breaks down the lignin and the wood turns gray and rough. The more coats of finish used, the greater the protection. Film finishes have to be sanded and recoated on a scheduled basis, somewhere in the 1-3 year range for good products; more frequently for some of the products that don't hold up so well. If the film finish is not maintained, and starts to crack and peel, it has to be completely removed and replaced.
The UV absorbers in clear finishes are sacrificial. They absorb the UV energy and are broken down in the process. Once the absorbers are spent, they no longer offer any protection and the UV goes to work on the surface of the wood and the finish itself. The lignin in the wood starts to break down which causes the finish to lose its adhesion and it develops cracks and starts peeling. At this stage, the only solution is to completely remove the old finish and apply a new one.
Penetrating finishes soak into the wood and preserve it without the look of a film over the wood. Penetrating finishes allow dirt to build up in the grain and don't shield it from wear. They don't require sanding, and don't crack or peel; but they do break down and/or wear away over time. They can be recoated after a good cleaning; generally with a power washer. They last about the same amount of time as a film finish.
All exterior clear finishes have to be re-coated on a regulary scheduled basis to renew them. The length of time between these maintenance steps depends on the product - some last longer than others.
The wood you used to make the chairs is an important factor. If you used a species that's naturally resistant to weathering, the finish is less of a concern. If the wood is suscepticle to damage from the elements, the protection the finish provides is more critical.
Paul
Furniture Finish Wizard
Paul, thanks for the lesson - very clearly presented. Do I understand you to say that UV protection requires a film finish? That is what I assumed, but it's always nice to challenge assumptions before accepting them. Or are there penetrating finishes that do offer UV protection?
The chairs under consideration will typically be exposed to direct sunlight for brief periods, during set-up and breakdown. In use they will be under a sunshade, awning, or tent most of the time, sometimes indoors. So I wonder how long it will really take for UV damage to occur/become apparent. The wood is QS white oak, with some walnut inlay.
Spar varnish really is low on my list, for this particular piece. The appearance would be awful when new (I tried a test piece - looked like I dipped it in plastic), and I would have to be the one to refinish them every three years or it wouldn't happen. Refinishing the carvings doesn't sound like fun, and sanding them is pretty much out of the question.
I still wonder about the penetrating epoxy. The white oak itself will hold up pretty well against sunlight, all I really want to do is make it as close to waterproof as possible (to prevent the mildew problem if they are improperly stored, which is what happened to the previous thrones).
If I apply any kind of film finish by itself, it will eventually develop cracks or dings through which moisture can get behind the finish, and then it's all over but the stripping. (In this regard a penetrating finish might actually be better because mositure doesn't get trapped behind it and it's easier to renew; but you pretty much have to commit to renewing it regularly.)
The epoxy manufacturer recommends using it as a primer - you soak it in, allow the volatiles to evaporate, then apply a spar or other film finish. The epoxy continues to cure after the finish is on, resulting in a great bond and preventing peeling... so they say! It is sold primarily for architectural use.
The other material I am curious about is the UV-cured penetrating oil. I've lost the URL, somebody here posted a link a short time ago, but it sounded intriguing; IIRC they advertised a 7-year performance guarantee."Everything should be made as simple as possible, but no simpler." A. Einstein
http://www.albionworks.net
I have gotten good service in exposure to sunlight fron Verathane outdoor Diamond Finish. It lasts about 2-3 yrs in exposures of 4+ hrs/day to bright sunlight in So. Calif. It is water resistant but even though advertised as having a UV blocker, the red and yellow pigments in my canarywood exterior shop door disappeared after 7-8 years while the walnut and mahogany elements of the outdoor table next to the door have not faded badly.
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