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Finish problem
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=46211		 Page 1 of 1
Author:  PeterF [ Fri Aug 21, 2015 11:49 am ]
Post subject:  Finish problem
I've been trying to get my ukulele finished for a few weeks now, but I've run into a problem I've never seen before. The lacquer (bolgers instrumemt lacquer, thinned 50:50) was going on fine, then suddenly one coat went weird. It blushed a sort of white hue with an oil-like sheen on the surface, but only in the areas that weren't in contact with something underneath. So you can see all the braces and linings through the surface.
It was rather humid the day I sprayed it and was raining hard outside, but I've sprayed in similar conditions before and it's been fine. I've just tried sanding it out and spraying again, but the same thing happened. Any ideas? I'm pretty certain I haven't contaminated it with anything, but I might have missed something.

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Author:  Hesh [ Fri Aug 21, 2015 12:03 pm ]
Post subject:  Re: Finish problem
It was too humid when you sprayed and why this did not happen prior is that your current build coats may be heavier.

It's that ole, darned blush thing.
Author:  meddlingfool [ Fri Aug 21, 2015 12:14 pm ]
Post subject:  Re: Finish problem
How does one recover from that?
Author:  John Cavanaugh [ Fri Aug 21, 2015 1:01 pm ]
Post subject:  Re: Finish problem
Lightly mist the surface with blush remover (or finely atomized lacquer thinner). May not always work, but when it does, it's like magic.
Author:  Goat Rock Ukulele [ Fri Aug 21, 2015 1:07 pm ]
Post subject:  Re: Finish problem
I have had something like that happen when I over thinned the lacquer. More likely too humid or you got some water oil in your line.
Author:  jack [ Fri Aug 21, 2015 1:12 pm ]
Post subject:  Re: Finish problem
add about 10% lacquer reducer to otherwise un-thinned lacquer, and maybe a bit drier day... the blush will come out
Author:  PeterF [ Fri Aug 21, 2015 1:33 pm ]
Post subject:  Re: Finish problem
Thanks, I thought it was probably something like that, although having the bracing appear on the surface was unexpected.
Goat Rock Ukulele wrote:
I have had something like that happen when I over thinned the lacquer. More likely too humid or you got some water oil in your line.

What do you mean by having water oil in the line?
Also, what is reducer? Is that the same thing as blush remover or retarder? I've never used any of them before - at work we just use standard lacquer thinner and don't really use any other chemicals.
Author:  Mike Collins [ Fri Aug 21, 2015 2:07 pm ]
Post subject:  Re: Finish problem
What John & Jack said.
Rough the surface up (320-to 600 grit'the grit size matters -if you have thin finish-use 600-the heavier ones for a thick finish.) then apply the thinner or blush remover.
NOT a heavy coat!!!
Wait.


Mike
Author:  jack [ Fri Aug 21, 2015 2:18 pm ]
Post subject:  Re: Finish problem
reducer = thinner
retarder= blush remover
Author:  Tim McKnight [ Fri Aug 21, 2015 9:56 pm ]
Post subject:  Re: Finish problem
Install a desicant filter at point of use as close to your gun as possible.
Author:  Trevor Gore [ Sat Aug 22, 2015 1:04 am ]
Post subject:  Re: Finish problem
PeterF wrote:
I've been trying to get my ukulele finished for a few weeks now, but I've run into a problem I've never seen before. The lacquer (bolgers instrumemt lacquer, thinned 50:50) was going on fine, then suddenly one coat went weird. It blushed a sort of white hue with an oil-like sheen on the surface, but only in the areas that weren't in contact with something underneath. So you can see all the braces and linings through the surface.
It was rather humid the day I sprayed it and was raining hard outside, but I've sprayed in similar conditions before and it's been fine. I've just tried sanding it out and spraying again, but the same thing happened. Any ideas? I'm pretty certain I haven't contaminated it with anything, but I might have missed something.

It's caused by spraying when the humidity is too high. The mechanism goes like this: Spray thinned lacquer on guitar/uke, solvent evaporates causing cooling, surface of lacquer drops below the dew point temperature, condensation forms on the cooled, sprayed surface giving the white blush. You see the brace outlines because the structure is thicker at those places, has higher thermal capacity, so doesn't cool as fast. With this mechanism, the blush forms very shortly after spraying and if you can get the instrument back into a low humidity, warm environment fast, the blush disappears entirely because the condensation evaporates off again. If you sprayed over the condensation and it does not disappear within 10 minutes of getting the instrument into your dry-room, all is not lost. If you can spray a wettish coat of your 50:50 mix within about 1 hour (but in dry conditions) the blush will most often release, though not necessarily immediately. If you can still see the blush the day after, you'll most likely have to sand it out.

I use a different product but a similar 50:50 mix. The cut-off RH is 60%, above which I don't spray. Where you live, it's a wonder you can spray at all! Tricks you might want to try are 1) warming the instrument before you spray, so it has further to cool before it gets to the dew point 2) Warming the lacquer. Here, I just leave it in the sun for a while. You may have a problem with that!

BTW, have they filled the holes yet?
Author:  PeterF [ Sat Aug 22, 2015 3:02 am ]
Post subject:  Re: Finish problem
Thanks for the explanation Trevor, that makes it a lot clearer.
Haha yes the sun can be rather lacking around these parts! With that in mind it's strange I've never had the problem before. I work at a small furniture making shop and we spray outside all the time unless it's raining, when we spray inside with all the doors open and not the slightest bit of humidity control! Does AC (acid catalyst) lacquer tend to not blush as much as nitro?
As to the holes, I think there are about 4000 more of them! :D

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Author:  Trevor Gore [ Sat Aug 22, 2015 7:52 pm ]
Post subject:  Re: Finish problem
PeterF wrote:
I work at a small furniture making shop and we spray outside all the time unless it's raining, when we spray inside with all the doors open and not the slightest bit of humidity control! Does AC (acid catalyst) lacquer tend to not blush as much as nitro?

I don't have enough experience with acid-cat to answer that one. However, I suspect most of your furniture has panels a lot thicker than guitar or uke panels and so greater thermal capacity, and that will make a big difference.

Regarding the holes, I checked them out again and came up with this and this.

Took me back to my youth when I lived in your part of the world...a pretty special time wherever you lived.
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