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Polyurethane coating on guitar: high spots

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Forum topic by GurfGurston posted 08-19-2018 04:44 AM 349 views 0 times favorited 4 replies Add to Favorites Watch
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GurfGurston

9 posts in 88 days


08-19-2018 04:44 AM

Hi All,

I am refinishing a guitar for the first time ever (doing luthier or woodwork) and I have a couple of high spots where the urethane was applied too heavily and ran over the side or puddled on the back. I decided to leave it and build up the coats a bit before sanding them down and have since learned how to properly apply the poly… so now I’m at around 20 coats wiped on with t-shirt cloth and it looks great except for these few run marks.

Is it okay to just wet sand them down or should I build up coats around them a little bit first? I’ve been scuffing between coats but it looks like now I’ll be removing a good bit of material. After that I guess clean with mineral spirits and a microfiber cloth then throw a few more coats over it??

Psalms 19:1 1 The heavens declare the glory of God; the skies proclaim the work of his hands.


4 replies so far

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mahdee

4006 posts in 1939 days


#1 posted 08-19-2018 01:10 PM

You may run into a few issues. One would be sanding over the layers which is very hard to cover with additional applications. The other is that it may take a long time for the high spots to dry especially if you have been putting additional coats over them. You will notice that when you go to wet sand it, the finish will roll out. Also, since the guitar is not handled like a coffee table, you may not need that many coats to properly protect the instrument; a thick coat may affect the sound. I would put it aside for a week or two before sanding and go from there.
Good luck

-- earthartandfoods.com

View Planeman40's profile

Planeman40

1272 posts in 2932 days


#2 posted 08-19-2018 02:46 PM

Try this trick.
Use a single edge razor blade (https://www.harborfreight.com/catalogsearch/result/index/?dir=asc&order=EAScore%2Cf%2CEAFeatured+Weight%2Cf%2CSale+Rank%2Cf&q=razor+blade) to scrape the run down level. Hold the blade at 90 degrees to the surface. The polyurethane must be absolutely hard and dry. I have scraped entire table tops, cabinets, and even musical instruments this way. Finish by wet sanding with a very fine black (carborundum) sand paper, then polish to a mirror finish with automotive rubbing compound (I use the
Turtle Wax brand from Pep boys.)

For what its worth, guitars should be finished with a couple of coats of shellac as a “primer” and sanded as above. Shellac is a superb sealer that sands well and is compatible with lacquers and other clear finishes on top. Or shellac can be THE finish! Great stuff.

-- Always remember: It is a mathematical certainty that half the people in this country are below average in intelligence!

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CaptainKlutz

524 posts in 1666 days


#3 posted 08-19-2018 06:46 PM

https://www.woodmagazine.com/woodworking-tips/techniques/finishing/film-flaws

FWIW –
Automotive painting world has a couple tools that might help:
1) ‘run shaver’: Tool to hold razor blade flat for shaving off top of a run
2) ‘nib file’: Is small file used knock dust nibs or bugs off top of finish. I have a Streck fine nib file and it works well to ‘plane’ top off a run or drip in poly.
There is no one perfect solution to dealing with finish imperfections. Depending on defect, may need to use both tools, then sanding to create final finish. Google/Amazon can show you many different versions of these tools.

-- I'm an engineer not a woodworker, but I can randomly find useful tools and furniture inside a pile of lumber!

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GurfGurston

9 posts in 88 days


#4 posted 08-21-2018 02:04 PM

Thanks for all the help fellas it worked beautifully.

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