Matching Finish on a Cured Poly Guitar

[SiGraybeard]

Hi,


I've been a member here for a while, but I think I've only posted a few times. I mostly just read and absorb, or look at build pictures. This time I'm looking for advise on finishing a repair on a junk guitar. The guitar is a Breedlove acoustic, finished with the cured poly-something that they do overseas.

I'm concerned about finish compatibility with that poly, except that it seems that solvents don't touch it. I've put acetone, mineral spirits and plain old denatured alcohol on it, and nothing bothers it. The only way I've marked it is to scratch it with a metal tool or sandpaper.

The finish I was thinking of using is Minwax gloss polyurethane. My main fear is that it doesn't adhere to the poly finish and flakes off. I'm not trying to refinish the body, just finish my repair, but it's inescapable that the finishes touch.

About 7 hours ago, I masked off most of the body and sprayed a small spot on the back. It hasn't shown any finish issues; no bubbling or anything, and while it isn't fully dry, it's like the tape and newspaper in terms of how sticky it is.

Question: am I way off track here? Do I need to lightly sand the poly finish to help the polyurethane adhere?

Is there an FAQ on finishes anywhere?


Thanks,
Bob

[jfgesquire]

MmmmPolyurethane is not like lacquer where it will melt into prior coats, and it allegedly can't be feathered very well either.

Everything I've seen about it says you have to scuff the old poly for the new poly to stick well. That means the whole top or the whole neck where you can go all the way to an edge or angle.


HOWEVER, there is an automotive poly method of finishing and feathering.

The idea is that you scuff an area larger than you'd end up spraying with the poly, leaving a scuffed edge that needs to be buffed out.... but before buffing hit the feathered edge of the newly sprayed poly with a blending agent to help level the edge even further.

The poly is a two part activation, called 2K and there is also a blending agent aerosol.

If no one else replies with any experience using this method on a guitar, I will take the plunge and give it a try.

The only downside is that these cans are about$20 each, and once the 2K poly can is mixed it's only good in the can for about 48 hours.

I recently fixed an end crack on a poly coated acoustic and really don't want to mask and spray the entire guitar's sides . . . there's too much to mask, and remove electronics and battery compartment, jacks, etc. It's a very dark rosewood and a super glue fill will leave a light edge showing through the finish. I need to sand it down anyway.

[John Arnold]

It probably is polyester, not polyurethane.
(My personal peeve about using the term 'poly', which means 'many'). It is not in any way definitive.
On polyester finishes, I sand with 400 grit and spray nitrocellulose lacquer on top. Adhesion is excellent. I cannot comment on the viability of using polyurethane, since I have never used it on a guitar.
BTW, what you are asking for is advice. 'Advise' is a verb.

[Ned Milburn]

Originally Posted by John Arnold

It probably is polyester, not polyurethane.
(My personal peeve about using the term 'poly', which means 'many'). It is not in any way definitive.
On polyester finishes, I sand with 400 grit and spray nitrocellulose lacquer on top. Adhesion is excellent. I cannot comment on the viability of using polyurethane, since I have never used it on a guitar.
BTW, what you are asking for is advice. 'Advise' is a verb.

Interesting, John... Have you done any other different finishes atop each other? I have a lovely test piece of brushed on water based "lacquer" that I French polished on top of. Looks great after 5 years, but has been humidity controlled in my workshop environment, so wonder how it'd weather in "guitarist world" here in Nova Scotia or elsewhere...

[Tony Done]

I've done near invisible repairs to minor dings in poly(whatever it is) finishes with CA glue, that is, there were no witness lines and it polished down to the same look as the rest of the finish.

[John Arnold]

CA repairs can be done on polyester, but there are pitfalls. One is when the polyester has a thin gloss layer over a sealer coat. If you cut through the gloss layer while leveling the CA, you may end up with a dull area that won't polish to a full gloss.

[Tony Done]

Originally Posted by John Arnold

CA repairs can be done on polyester, but there are pitfalls. One is when the polyester has a thin gloss layer over a sealer coat. If you cut through the gloss layer while leveling the CA, you may end up with a dull area that won't polish to a full gloss.

Good advice, thanks. I've had that happen recently when trying to polish a satin poly finish