Why did I get these crazy cracks after spraying gloss acrylic on a headstock?

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by NoahL, Mar 10, 2008.

  1. NoahL

    NoahL Member

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    Just tidying up a beat-up headstock on a low-priced Hamer. I sanded it and hit it with a thin coat of acrylic, and it looked good. Steel-wooled it two days later and did the same. Looks great, until you look close and see the crazy little cracks. What would have caused that? My little workroom can get cold at night, and I probably did this around midnight, before the heat went down to about 57 or so. Could that be it? What else? Spray too thick? I'm sure the previous coat was quite dry.
     
  2. doublee

    doublee Member

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    if you didnt sand it down to the wood first there may have been an incompatible layer of (nitro? poly?) underneath. The temp should have been fine...
     
  3. NoahL

    NoahL Member

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    I kind of assumed that the sanding and steel-wooling would create a good substrate, as when I de-gloss oil-based paint on the wall with TSP and the latex paint adheres just fine. But maybe that's not sufficient.
     
  4. David Collins

    David Collins Member

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    Not sure if this would have aggravated anything, but steel wool is very oily stuff. It's great for smoothing shellac between sessions, but I don't think I'd use it with acrylic lacquers. It may have have nothing to do with the cracking though, as thickness, different expansion rates of underlying layers can make much more sensitive to cold checking, who knows. There are lots of things that can be incompatible in finishing.
     
  5. NoahL

    NoahL Member

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    thanks. for these cheap guitars i monkey with, i think i need a spray-on poly from now on. this one has cool relic-ing now. i don't mind it. ;-)
     
  6. patpark

    patpark Member

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    oil in the steel wool reacted with the paint?
     
  7. NoahL

    NoahL Member

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    i think he means it left an oily layer on the headstock and the acrylic lacquer didn't settle in/on well enough.
     
  8. Mike9

    Mike9 Supporting Member

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    I wipe the work down with Naptha before spraying. It evaporates fast and leaves no oily residue. I also tack right before and inbetween coats.
     
  9. Rosewood

    Rosewood Member

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    Be careful using tack cloths, especially between coats, it can cause all kinds of problems. The main thing is don't press hard on the tack cloth so it doesn't leave any residue.
     
  10. syxxstring

    syxxstring Member

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    Sounds like either a contamination through residue issue to me or the temperature.
    Like others have said clean and tac before spraying. And most finishes like to be at least 70 for the solvents to play nicely and get out of the way so things can cure smoothly.
     
  11. Mike9

    Mike9 Supporting Member

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    Another trick is to let the can sit in warm water to bring the temp of the contents up. Just remember to wipe it down before spraying. I don't use fresh tacks on lacquer - I use ones that have been open for a while just sitting in a baggy. They are less tacky that way - you're really just doing a light wipe for dust.
     
  12. 57special

    57special Silver Supporting Member

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    don't use steel wool! Having said that, it might not be the reason for the cracking.
     

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