Touching up/repairing old polyurethane?

[rob.]

I have a large chip in some polyurethane that I need to repair, I need to fill it in, the original has yellowed over the last 40 odd years to a golden honey colour. If I fill with new polyurethane will I have to tint it to get it to match the existing? Im not after an invisible repair, just want to minimise the damage and make it look less of a mess.

Also, can I thin polyurethane so it will leech under some finish thats still attached but lifted from the wood?

Any help would be greatly appreciated.

 

[MikeG.]

I'm pretty sure that new polyurethane doesn't adhere to old polyurethane. Unfortunately, I doubt there is a good repair available to you other than stripping the whole thing.

 

[profchris]

Are you sure it's polyurethane? Many guitar finishes are catalysed polyester (very brittle if it flakes).

The standard repair for polyester is CA glue drop fill (search online for tutorials). Not sure how you'd tiny it though.

 

[mbartlett99]

Polyurethane will stick just fine but you'll have difficulty getting it to leech under. The boys on deck have to do this all the time when they screw up with something heavy; build up the coats then buff it as best you can. Getting a colour match will be very hard.

I thought guitars were often nitro cellulose or even shellac though?

 

[rob.]

Older ones would be nitro, also some modern ones but not many, shellac is more an acoustic guitar finish as far as I know and not really mainstream commercial.
Polyurethane or Polyester, I really thought they were the same thing, I had a brief look and it would seem they arnt! I would assume this is Polyester, but thats an assumption , I dont know for sure.


If Polyester and I used glue I wouldnt be able to tint it, would I even have to tint it? Does polyester age and yellow like polyurethane?

 

[profchris]

I'm a long way from any class of instrument finisher, but if the guitar is not valuable ...

Try a drop of CA glue on the raw wood where the nut was. The headstock wood has changed coliur with light exposure, so if the result is close but a bit paler then maybe all it needs is the flaking removed (glueing it back down will look awful) and then clear CA drop fill. Even if not a perfect colour match it should look better than the flaking. I'd try just a spot at the edge of the feedstock to see.

Does the finish dent with a fingernail? If rock hard, it's polyester.

 

[rob.]

Its a valuable guitar in its rarity , money wise not so, it was built by Matsumoku between 72 and 74, from what I gather its one of a thousand built however this is unconfirmed, there are very few references to it on the internet so information is scarce as are other examples.


This is why I dont want an invisible repair, but I do want to keep it tidy.

What is CA glue? I have read about using super glue to drop fill scratches etc but I was hoping that the existing finish while still not attached to the wood could be made good by leeching some finish between it and the wood it detached from.

 

[profchris]

I make acoustic instruments and finish using shellac, so this is all from reading.

1. Cured polyester is inert, so fresh finish (of any kind) won't melt into it. Chemical strippers wont remove it. If you glue it down you'll get a refractive boundary (like ice on a pool) which means light reflects differently. So the glued down patch sticks out like a sore thumb.

2. The best repair is a finish with similar light transmission/reflection characteristics. Poly is a kind of plastic.

3. CA = superglue, and cures to clear acrylic plastic if you apply it right. The technique is building up thin layers, scraping back, and then polishing to match.

4. Thick layers, accelerants, moisture in the air can make it foam white - start again!

5. It's a slow job, you need to be good with a scraper (usually razor blade) to level it to the surrounding finish without damaging that.

If valuable, at least ask a luthier skilled in this kind of repair for a quote. It takes practice to achieve a near invisible result.

 

[rob.]

Thanks buddy, I am not after an invisible repair, its a part of the guitars aesthetic personality now, it is what it is. So, to make it less obvious or messy I would need to remove the separated finish? Apparently, I have been told, Matsumoku used Polyurethane and not polyester, If it is indeed Polyurethane it should be quite flexible and not brittle , right?

 

[profchris]

If polyurethane, certainly not brittle like glass. Will it dent with a thumbnail?

I suspect that new polyurethane is what to apply, others will know better than me how to minimise witness lines.