UV cure finish issues

[Roxy64]

I recently started using the Simtec UV curable finish and ran into an issue. I had tried spraying it over a recently dyed sunburst and noticed a slight color shift of the brown's to greenish hue so i tried spraying a spit coat of dewaxed shellac to seal the dye prior to spraying the UV cure. upon curing the finish, it appears that the heat from the mercury vapor cure lamp causes the underlying dried shellac to soften or melt causing adhesion issues and bubbles in the UV finish. has anyone had experience using this type of finish over a dyed surface? I'm using transtint dyes in alcohol. I'd like a seal coat that's going to be compatible.

any suggestions would be helpful.

[rcc56]

I'm just an old-time repairman, not a builder, so you can take what I say with a grain of salt.
I've seen a lot of stuff come and go-- new finishing materials, new glues, the latest kiln-drying trend, etc.
In most cases, I have found it best to stick to materials and techniques that have withstood the test of time.

The best traditional finish materials take time to cure, but we know what to expect from them. There's always a new product that comes along that is meant to reduce curing time, but we never know how well it will work or stand up until it has been in use for quite a few years.

I believe in nitrocellulose lacquer, traditional spirit and oil varnishes, aniline dyes, hide glue, and Titebond Original.
Trans-tint dyes have now been in use long enough to be considered to be reliable. Some of the modern "conversion varnishes" seem to be standing up well, but at least some of them are less durable than nitro.

I've looked at your website, and you build very nice instruments. I would think it worthwhile to take the extra time to finish those instruments with nitro or good old-fashioned varnish, even though it will take longer for those finishes to cure.

I've torn my hair out a few times over drying problems and "greening." Most of those problems went away when I changed over from denatured alcohol to pure grain alcohol from the liquor store to mix my dyes and shellac.

I can't help you with UV finishes. I do know that UV and IR finishes and glues have had a speckled history since they first started to appear 50 years ago.
If you prefer to work with one of the new materials, I recommend that you test them on scrap until you are reasonably sure that they will work on an instrument.

Good luck. Finishing is perhaps the hardest part of the craft for me.

[j. condino]

'Lots of good advice in the above post...

Since you are into new territory and mixing multiple items, your best bet is to put everything aside and run a series of very methodical, quantitative tests on each of them in combinations, basically building up a matrix of all the possible options so you can best answer your own questions. The numbers won't lie. Do that first, and then move to the working instrument.

I've used a lot of UV cure in large production settings, and small light boxes to help cure out oil varnishes on violin family instruments, but I've never once desired a UV system for guitars or mandolins. I'm curious what you are looking for with this new material?

[sunburst]

It would seem that UV-cure sealer would be compatible with UV-cure finish. There's no guarantee, but it's worth looking into. Usually finish manufacturers will tell you what is and is not compatible, but they have to cover their @s against potential law suits, so sometimes they will tell you something will not work when in fact it will.
The rule of thumb that shellac is compatible under or over nearly anything no longer applies.
At Troublesome creek, before I left, they were going to UV-cure sealer under Seagrave lacquer. It worked for adhesion and time saving, but of course we do not yet know long term.

[Roxy64]

Thanks for your response. your suggestion is exactly what i am doing. I had called the manufacture about spraying it over a thin seal coat of dewaxed shellac and he didn't have any comment which prompted me to try it out.

[Graham McDonald]

There have been numerous reports on luthier discussion forums about difficulties with UV cured finishes. A couple of recurring themes are incompatibility between sealer and top coats from different manufacturers as well as seal coats not curing properly in the pores of open pored timbers like mahogany. I get the sense the UV cured finishes can be great for a company like Taylor guitars making dozens (100s?) of guitars a day, but for small production workshops it is probably not worthwhile.

Cheers and a Merry Christmas

[mirwa]

I recently started using the Simtec UV curable finish and ran into an issue. I had tried spraying it over a recently dyed sunburst and noticed a slight color shift of the brown's to greenish hue so i tried spraying a spit coat of dewaxed shellac to seal the dye prior to spraying the UV cure. upon curing the finish, it appears that the heat from the mercury vapor cure lamp causes the underlying dried shellac to soften or melt causing adhesion issues and bubbles in the UV finish. has anyone had experience using this type of finish over a dyed surface? I'm using transtint dyes in alcohol.

I spray with uv, it is one of the easiest and best finishes around, I would not use shellac as a sealer coat, use the uv sealer then follow up with uv crystal clear

Uv sealer coats are a great foundation for any finish, i have top coated over uv sealer with nitro, 2 pack, varnishes, acrylic lacquers etc

Steve

[Andy Miller]

I've sprayed Simtec UV clearcoat over the top of Mohawk nitrocellulose lacquer quite a lot after doing airbrush touchups over repairs. I don't recall ever having issues with things coming loose or bubbling when I cured it.

[Richard500]

There are a few common ways to get enough UV for curing: some with more unwanted other frequencies attached, like the old ‘sunlamps’ which were just overvoltaged tungsten lamps with thin glass envelopes and were mostly heat lamps with a little UV. What I used for photolithography was just a bare quartz halogen lamp, and later, an old halogen outdoor floodlight which had (by itself) detached its cover plate. If overheating is an issue, the idea is to use a source that leans more toward the useful UV. I don’t think diode sources are powerful or cheap enough just yet for big objects, but would be ideal. What do you people use?

[sunburst]

I'm out of my area of expertise here, but...
Whatever the UV cure sealer we used at TC was, it cured in sunlight in 15 minutes or so. It would then sand to a dry while powder with no loading of "freecut" sandpaper. It was labeled as being 100% solids, we could squeegee it into the pores of open grained woods (like walnut) and it would cure with a level surface to sand and spray.

[amowry]

Hi Gary--I've certainly seen issues with shellac under various modern finishes, and I wouldn't recommend it as a sealer under anything else unless recommended by the manufacturer or tested extensively. Shellac is often touted as a universal sealer, but that is no longer the case. I saw something similar to what you describe with both RoyalLac and Cardinal lacquer, where the bond with the shellac sealer failed and caused bubbling. Dyed sunbursts present a problem with a lot of modern finishes because you have limited options for sealing them. That's partly why a lot of guitarmakers put the burst in the finish, but of course it's a completely different look that way.