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Polyurethane won't cure

Discussion in 'Finely Finished' started by Twentyten, Jan 20, 2021.

  1. Twentyten

    Twentyten TDPRI Member

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    Hey guys, so I refinished a guitar for the first time recently, and it was only 35 to 50 degrees in the garage at any given time. I used Minwax semi gloss polyurethane, which I thinned to be able to wipe it on. The guitar looks good by my standards, but the problem is that after a little more than a month I can still dent the finish with my fingernail. Will it cure eventually? Or do I have to start over (again)?
     
  2. schmee

    schmee Doctor of Teleocity

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    Ugh, I've had that happen with higher temps even. I refuse to use Minwax. I dont know why it does that. Best I can say is bring it in the house near a heater vent. The body I did never did get real hard though. You may want to just strip it. If you want to do a quick and dirty finish like that I hand brushed this stuff and it cures fast and levels out great.
    [​IMG]
     
  3. mfguitar

    mfguitar Tele-Afflicted Silver Supporter

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    What type of Minwax poly did you use and what did you thin it with?
     
  4. Twentyten

    Twentyten TDPRI Member

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    Dang... thanks for the tung oil tip. I'll probably at least give the poly a chance until Summer.

    I used oil based Minwax fast drying polyurethane in semi gloss. The paint thinner was just Klean Strip paint thinner
     
  5. WalthamMoosical

    WalthamMoosical Tele-Holic Ad Free Member

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    Welcome to TDPRI, by the way!

    The minwax poly finish I did last summer took quite a long time to cure. It was a couple of weeks before I felt that I could handle it casually without easily dinging it up--but note, this is a paulownia body and just a "learning" project anyway so I was not that concerned about it. If it has spent that month at 35-50 deg then give it at least a few months in the warm before giving it up.

    Good luck!
     
  6. Twentyten

    Twentyten TDPRI Member

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    Thanks! And thanks for the advice, it gives me some hope for it still. It's been in a closet at 60 for that month, but I'll put it in near a heater for another 2 or 3 and see what happens
     
  7. WalthamMoosical

    WalthamMoosical Tele-Holic Ad Free Member

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    Ooops ouch I meant to type "at least a few weeks ..." I think if after several months at normal temperature it weren't pretty hard I'd be pretty disappointed.
     
  8. Twentyten

    Twentyten TDPRI Member

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    Sounds like I might be disappointed in the near future. Live and learn I guess
     
  9. mfguitar

    mfguitar Tele-Afflicted Silver Supporter

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    Your problem may be in the paint thinner, while you can clean up with thinner I rather doubt that it suggest that you thin the product with a standard thinner or if you should thin at all. Modern oil finishes are really modified oils, depending on the container size you purchased, gallons have a different formula than quarts. It may be the poly finish is delaying the evaporating of the thinner you used. I would double check on the label to see if they recommend thinning but I do think your finish will harden eventually
     
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  10. Peegoo

    Peegoo Poster Extraordinaire

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    Uncured poly is sticky.

    I think your issue is that it's thinned and wiped on--which makes the cured finish really thin. It's as hard as it will get, but the wood beneath it is really soft and provides no support.

    Cured poly is not hard like glass. It's only as stiff as whatever you apply it to. Coat a piece of printer paper with it and you can still roll up the paper after it's cured.
     
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  11. dogmeat

    dogmeat Friend of Leo's

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    yeah... I doubt it will ever cure completely. I've used the Minwax wipe on quite a bit in the last 2 years (complete set of kitchen cabinets, 7' island counter, 2 guitars). wipe it on generously at full strength with the minimum motion. don't hit it more than 2 or 3 times, then leave it alone. all this work done in my garage in AK in the winter (mostly) temps there in the low 50s. for an applicator I used a replacement element for applying floor sealer. I got it at Lowe's, about 8x12 inches(?) maybe BlueHawk brand. one fuzzy side, one foam. the fuzzy side worked best. cut off a square, use it, stuff it into a baggie. good for a couple days then it gets too stiff
     
  12. Twentyten

    Twentyten TDPRI Member

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    So if the wood is the problem for me, what should I use for a finish to make it as hard as from the factory?
     
  13. Boreas

    Boreas Friend of Leo's Silver Supporter

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    What kind of wood? What kind of filler? What kind of primer? How did you apply the poly? How much did you thin it? How long between coats? Does it still smell quite a bit, indicating it is still drying? Did you use it on a maple neck? Is that any harder?

    I used the identical poly on a maple neck, and it was very hard within 3 days. I gave it a week just in case. No primer. I did not thin it and used thin coats. As I recall, 4 coats - dried 1 day between coats. Applied and dried at room temperature.

    There are many variables that effect finishes. The more information you can provide helps narrow down the reason for the problem and possible remedy.

    Many factory finishes today use catalyzed, 2-part poly which makes it similar to epoxy. You aren't likely to get the same hardness with typical consumer-grade finishes.
     
  14. stratisfied

    stratisfied Tele-Holic

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    Assuming you brought it indoors to dry. a week is plenty of drying time. What was the polyurethane wiped over?

    I would remove the finish and try again with a different topcoat.
     
  15. Twentyten

    Twentyten TDPRI Member

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    I did a sunburst with ink and put the poly over that.

    Also, I think its an agathis body, so maybe that has something to do with the denting like somebody said above.
     
  16. dogmeat

    dogmeat Friend of Leo's

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    yeah... sounds like it's "Pauloweenie" wood... about 1/2 step from balsa. nice & light, but self-relicing. touch it and theres a dent. thats a different problem, and not related to the finish. get rid or the gooey top coat and put on something else
     
  17. Jim_in_PA

    Jim_in_PA Friend of Leo's Silver Supporter

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    Finishes pretty much always have an acceptable temperature range right on the container. Cold temps will absolutely affect curing. And remember, it's not just the air temps that have to be within the range; so does the workpiece and the finish. Get the project into a "room temperature" location to see if it will fully cure. Oil based finishes can take a long time for that even under ideal situations...
     
  18. Twentyten

    Twentyten TDPRI Member

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    Thanks for the help guys. So my plan is to wait a few months until the weather is warm enough to refinish the guitar, and in the meantime I'll keep warming it up in front of a heater. By the time the weather is good enough I should know if it'll ever get harder or not. Even if it doesn't, maybe I'll just play it and let it look like a relic.
     
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  19. Boreas

    Boreas Friend of Leo's Silver Supporter

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    Do you have a maple neck? Does that finish seem harder? Will it dent with a fingernail? If it does, then the finish may be part of the problem. But if it acts a lot different, it is primarily the soft body wood.
     
  20. adjason

    adjason Poster Extraordinaire Silver Supporter

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    weird I too think the problem may lie in the thinner used. did you put on lots of coats? In my experience poly gets pretty hard in a couple of days. rags left to dry get pretty stiff too
     
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