UV Cured Finishes

There's a lot of chatter about "UV Finishes" here with little apparent understanding of what they are.

"UV" is not in and of itself a finish. "UV"...Ultra Violet light...is a means of curing modern polyurethane and polyester finishes, and is used instead of the more conventional addition of a catalyst to the base resins. The main advantage of UV is the speed of cure...it can be under a minute from out of the spray gun wet to cured when the coating is exposed to the right wavelength and intensity of UV light. The secondary advantage is that you don't have to mix resin and catalyst together, and in fact, the urethane or polyester can be pumped from a drum and the gun can be left unattended overnight...the finish won't cure in the liquid lines or gun.

As a uke owner, you cannot tell the difference between catalyst and UV cured finishes. Catalyst cured polyester MAY be a tiny bit harder than the same material UV cured according to guitar maker Tom Anderson. In fact, the same exact basic resin formulas can be cured either with a catalyst (MEKP in the case of polyester, a more complicated formula for urethanes) or with the addition of a UV cure initiator (made by CIBA Chemicals and others...) and then exposed to UV. Tom, in fact, does some of his finishing as a "dual cure"...he adds MEKP to the UV initiated polyester, does a quick UV cure, and then has the additional harness of a 24 to 48 hour catalyst cure.

Bob Taylor advised me not to bother with UV curing finishes unless I was hitting about 100 instruments a month. Finish cure is not a real bottleneck under that number. It's razzle-dazzle; it's a great science fair project, but it's not a money maker unless you're doing major production. Frankly, spraying five instruments a day isn't really a big deal, and with the cat cured stuff, you can go start to cured top coat in five days...so you've got maybe 25 instruments hanging in various states of drying and cure at any given moment. Not a big deal.

There are UV and catalyst curing polyurethanes and polyesters, and because each has its advantages and disadvantages as materials, they can also be used together. I, for instance, use cat-cured urethane sealers under cat-cured polyester build and top coats, and I sometimes do sunbursts, translucent, and opaque colors with cat-cured urethanes between polyester build coats and polyester top coats.

And some of us use nitro lacquer top coats over polyester fill and build coats. This is quick and helps to avoid long term finish shrinkage into the pores of the wood. Note that UV is of no use in curing nitrocellulose lacquer...as my friends at SCGC expensively learned...

To make things even more complicated, there are UV curing polyester pore fillers that can be used over sealer wash coats and under just about anything. The disadvantage is that you must isolate polyester from Western red cedar or any rosewoods as the oils in the wood inhibit the cure of the polyester. Also, sometimes the UV light will not penetrate down into the pores, and you wind up with polyester jelly with a cured finish on top.

UV cured urethanes and polyesters are available in gloss or various degrees of satin sheen.

I get my urethane isolater/sealer and my polyester build and top coat materials from Simtec. WLS is a great supplier of urethane colors.

So, to wrap it up here...UV is simply a method of curing modern finishes; it is not a finish unto itself. I've seen a lot of luthiers and small manufactures be seduced into the UV cure thing...but given the cost of setting up for it, I can hardly see how it's worth doing unless one is building a whole lot of instruments. The cured finish is no better than that done with a catalyst.

Thanks Rick. I've often wondered about the differences in the finish coats. Nice to know more from someone who works with finishes all the time.

Very informative Rick and, as always, cleanly written.

I'll admit that I had no idea what UV finish really meant, so thanks for the education.

Thanks Rick,

I always assumed UV was a better finish since Taylor was doing it but you know what they say about assume.

Very informative and interesting Rick. I'll probably never in my life finish an instrument or even be greatly concerned about what is on the ones I buy as long as they sound good and are smooth enough to not give me splinters but I still find the tech stuff very interesting.

I too have to confess to having assumed that there was some sort of mojo in UV curing (I did at least know it was the curing process, though not much more than that). I suspect I believed that because it seemed like it would be expensive and therefore there would be no reason to do it if it wasn't bringing some advantage. I guess the advantage is in keeping the product moving through a limited-size shop when production is high enough.

John

thanks for sharing that Rick

Very helpful Rick. I've learned a great deal over the past two years from your posts.
I hope you are storing these for your book on rock music history, RTurner autobiography, the science and practice of luthiery, and a bunch of other topics.

Another consideration is that UV finishes release less anthropogenic volatile organic compounds in the curing process. Much better for the environment and the uke builder.

Timely thread. I just picked a few "Bridge Tie-Guards" from the Southcoast strings site. The instructions mentioned allowing new synthetic finishes 6 months to gas out before using the static cling protectors. French polish and UV finishes are cleared to use immediately.

So it sounds like UV finishes don't off gas much.

rar jungle said:

Another consideration is that UV finishes release less anthropogenic volatile organic compounds in the curing process. Much better for the environment and the uke builder.

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Yes, I was wondering the same thing. Can you talk about the environment and VOCs? I think this is the big win with UV, though I am no luthier.

Catalyst cured polyurethanes and polyesters are essentially VOC free in 48 hours, and the VOCs in the liquid are absolutely minimal. We put clear pickguards and classical bridge style "tie guards" over catalyst-cured polyester as quickly as 72 hours after it's been sprayed and never have a problem. I would NOT do that over nitro lacquer or French polish. In other words, my advice is 180 degrees opposite what you just said. It takes days, weeks, years for the thinner to evaporate from lacquer and days, certainly for it to evaporate from shellac.

I do not thin my urethanes or polyesters, and they are close to 100% solids, so the environmental factor is pretty green.

Once again, the basic resin is exactly the same for UV or catalyst-cure-initiated finishes. It's just the means of catalyzing the polymerization reaction. I could mix in some of the CIBA UV initiator with the same polyester that I normally MEKP cure, and even put it out in the sun for a few hours and get a cure.

BTW, one of the major industries using UV cured polyester around here in Santa Cruz is the surfboard making business. There are even 'board ding repair kits with UV cure poly that are designed to work in sunlight.

How many a month to justify a robot arm?

AndrewKuker said:

How many a month to justify a robot arm?

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Hey Andrew again don't be so modest..

you put on a lot of the beautiful high gloss finishes on the Ko'olau ukuleles in the day..but it did look like an intensive process getting the finish to look so nice..

Rick does the nicest polyester gloss I have ever seen..

but your nitro lacquer was up there with the best of them..and your French polish was way up there too..

Well, I also saw robotic spraying at Martin, but these guys are making 200 instruments a day!

Actuall, the robotic arms can be had as industrial surplus pretty inexpensively...well under $20,000.00. But then there's the programming which doesn't come cheap. National Resophonic got a robot arm that they use for UV curing finishes using a "hand held" UV light unit.

Another issue with UV is that it's dangerous...as in exposure to the light. Bad for eyes, bad for skin, so you either have to be well suited up with a full face shield and UV blocking mask lenses or do the UV in a closed cabinet as per many of the guitar manufacturers.

OK, time to go shoot a couple of sunbursts...by hand!