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Pinhole Hell

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by stratovarius, Nov 16, 2005.

  1. stratovarius

    stratovarius Supporting Member

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    Gee, I'd like to be able to shoot a coat of nitrocellulose without ending up with any pinholes. :(

    I've learned a lot and I'm getting better and better but I just can't seem to beat this problem completely. Of course it doesn't help that I live in Dallas and am trying to do this in 100 degree tempertatures with 80 percent humidity!

    Now that the weather is cooling down I want to try it one last time. I am using thinner and thinner mixtures as I get to the final coat, and yes, I am using more and more retarder as I go. The coats are looking really good but when start sanding there they are - tiny little pin holes! Unfortunately, I learned the hard way not to try to sand them all out.

    I am wondering if the heaviness of my passes has anything to do with the results? I am using a cheap HVLP gun. Does anyone have any suggestions?
     
  2. Turbozag

    Turbozag Senior Member

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    Usually, pinholes are caused by some contamination such as silicone or oil etc... (pray it's NOT silicone, that stuff is hard to get rid of)...

    I'd try cleaning everything REAL good, and use the best quality nitro you can find. There is some bad stuff out there...
     
  3. AS193

    AS193 Member

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    Make sure your airlines are not contaminated with oil from your compressor, and that you have both a filter/trap at the tank, and right before the gun.
     
  4. stratovarius

    stratovarius Supporting Member

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    Thanks for the responses. I am using a pretty cheap HVLP sprayer so maybe that's the problem? There aren't any filters on this thing at all, but these things are different than traditional spraying rigs. There isn't really a compressor - it's more like using the exhaust on a vacuum cleaner.
     
  5. AS193

    AS193 Member

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    It may be drying too fast. The turbine HVLP's send out (somewhat) heated air, and that's what may be happening. Try using a little retarder in the lac.
     
  6. stratovarius

    stratovarius Supporting Member

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    This is one of the things I've been suspecting. I've been using a lot of retarder and it still seems to dry really fast.

    What is a good rule of thumb for drying time (won't take a fingerprint)?
     
  7. AS193

    AS193 Member

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    Depends on the lacquer,ambient temperature, and humidity.
    Solvent type lacquers dry really fast, to touch in 10-15 min.@ 70+deg.
    Acrylic lac's take a few minutes longer, but still relatively fast dry.
     
  8. RL in Fla

    RL in Fla Member

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    Another thing to consider is whether it's heating up as it dries .
    Wood warming up = expansion = airbubbles .

    A pretty good Aussie wood worker tipped me to that one 193 ;)

    I was getting 'em even with wipe-on Poly until I started to finish in a slightly cooler environment than where it had been sitting , or let the shop warm up a couple degrees , and then cut the AC back down a notch so it'll be cooling down as it dries . May be irrelevant in this case , but who knows ? I never have humidity problems unless it's drying time on slow stuff , and we've got it with a capital "H" :(
     
  9. AS193

    AS193 Member

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    Hey RL, I'm sure you're much more familiar with the effeects of humidity than I. :jo



    Good to hear from you :)
     
  10. Vince

    Vince Member

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    When you say they reveal themselves when you sand makes me think you have trapped air bubbles in your material. When you sand the tops off the bubbles, it looks like a pinhole. That's usually due to a bad material flow to atomization ratio. Could also be water or oil in you lines, but more than likely you need less fluid and more air pressure at the air cap.
     

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