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How do you deal with repairs to catalyzed poly finish?
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Author:  Freeman [ Tue Sep 27, 2016 1:53 pm ]
Post subject:  How do you deal with repairs to catalyzed poly finish?
About 90 percent of the guitars that I get to work on have modern catalyzed polyethylene or polyurethane finish. I try to avoid disrupting the finish at all but sometimes I need to sand the repair or otherwise remove some of it. Headstock cracks are a common problem but it can be almost anywhere where the finish is removed to bare wood.

With shellac and nitro I know what to do. But lets say I've got a headstock repair with a backstrap on a newer PacRim guitar that I know is poly. It probably has some stain or dye in either the wood or the finish itself. What products, steps and procedures would you use? What if its a shiny black finish (or some other color)?
Author:  fumblefinger [ Tue Sep 27, 2016 10:08 pm ]
Post subject:  Re: How do you deal with repairs to catalyzed poly finish?
Of course, small dents/dings in clear coat can be fixed using standard drop fill techniques. https://www.youtube.com/watch?v=rTVScFJoe24

When I've done neck breaks in the past, I've used CA to fill small gaps, sanded and shot color. However.....
Now that I have black CA (S-M), if that's the color of the neck, I'll simply fill any the cracks with it. After the glue up, of course. Scrape, then sand and then buff. Just finished one before I had the black CA. Got it all ready to paint and on a whim, buffed the area. Jeez! It looks great. Well except for the wood that is just peeking out of the crack. Rather than shoot the neck, I'm going to dig out the clear CA and fill it with black and repeat the smoothing process.

If you can find a gloss Duplicolor that matches or is close enough, it generally buffs up nicely. I recently did a small repair on an artic white firebird. The Duplicolor matched surprisingly well. In fact I had to work a bit to get the Epi polly to match the gloss level.
Author:  B. Howard [ Wed Sep 28, 2016 7:31 am ]
Post subject:  Re: How do you deal with repairs to catalyzed poly finish?
This depends a lot on the original color, type of finish and the type of damage..... Anything without a crack in the finish can be filled with CA. Any type of crack in the existing finish will show as a line unless it is camouflaged under a new coating. I have access to most of the original type coatings and typically use them, but those who do not have full featured finish shops at their disposal are going to find options limited.

Simple black can often be filled with burn in stick and then sealed over with CA. As the color gets lighter this can become less of an option as the edges will show more. CA does not tint well as it typically kicks off with the addition of any type pigment but you can easily make colored base coat from lacquer or epoxy to match and then again seal over with CA and buff out.
Author:  fumblefinger [ Wed Sep 28, 2016 8:08 am ]
Post subject:  Re: How do you deal with repairs to catalyzed poly finish?
B. Howard wrote:
This depends a lot on the original color, type of finish and the type of damage..... Anything without a crack in the finish can be filled with CA. Any type of crack in the existing finish will show as a line unless it is camouflaged under a new coating. I have access to most of the original type coatings and typically use them, but those who do not have full featured finish shops at their disposal are going to find options limited.

Simple black can often be filled with burn in stick and then sealed over with CA. As the color gets lighter this can become less of an option as the edges will show more. CA does not tint well as it typically kicks off with the addition of any type pigment but you can easily make colored base coat from lacquer or epoxy to match and then again seal over with CA and buff out.


The Master speaks!
Listen to Brian. There is a reason other luthiers bring their things to Brian for finish work.
Author:  Freeman [ Wed Sep 28, 2016 5:20 pm ]
Post subject:  Re: How do you deal with repairs to catalyzed poly finish?
Thanks for the comments, guys, but lets approach the question a little differently. First, I'm a amateur repair person trying to hone my chops - as a result I do a lot of repairs that Brian or others might pass up. I also don't promise the world and don't charge a huge sum (I loose money on every repair that I do). For whatever reason I've done 12 or 15 headstock breaks in the past year or so - mostly I am happy with them structurally, can get them smooth enough to not present playability issues but mostly the cosmetics suck.

So, lets say I've got the usual complement of amateur tools and supplies. I have nitro and water born lacquer (KTM-9 but it could be others) and I'm comfortable shooting both. I've got shellac, vinyl sealer, nitro in both cans and rattle cans, lacquer "sanding sealer", TruOil. I've got an assortment of liquid dye (compatible with both water and alcohol) and paste filler in brown and redish brown. I've got the usual solvents - lacquer reducer, DA, acetone, medium and thin CA (including black) and a couple of slow setting epoxies (StewMac and Zpoxy). I've got a small HPLV that I'm comfortable with and an air brush that I've played with.

I think I can do the repair OK - that really isn't the issue. If necessary I can do a back strap in the same wood as the neck. I can work with HHG (as long as conditions are perfect), Titebond (most of the time) and I've used slow set epoxy but am currently rethinking it. So, here are twoa very typical breaks

Image

Image

Lets say it is a $500 PacRim guitar, I know the finish is something other than nitro (thinner doesn't touch it). I do the glue up - HHC or Titebond - and decide to put a back strap on it. Contour that into the neck and head and I'm ready to finish. What I've got is bare mahogany - neck and patch, finish that has been feather sanded into the repair, shiny finish on the rest of the neck. There is a glue line at the repair and a few small voids to fill.

What I am asking is the steps to try to make this look as good as I can with what I've got available. Pore fill, stain, shellac, what kind of finish material (and how to apply), feathering it into the stock finish, sanding and buffing.

I'll leave the questions about colors for right now - lets just fix some plain old mahogany
Author:  jfmckenna [ Thu Sep 29, 2016 10:07 am ]
Post subject:  Re: How do you deal with repairs to catalyzed poly finish?
Shellac can be tinted and will stick to anything. Might not be the expert way but it will work.
Author:  B. Howard [ Fri Sep 30, 2016 6:56 am ]
Post subject:  Re: How do you deal with repairs to catalyzed poly finish?
In that type situation with what you have available I would proceed something like this;

Prep sand new/bare wood to 220 and stain to match if necessary. Use medium CA as a sealer & pore filler by wiping on thin coats with a piece of paper towel. Sand this and an extended area around the repair with 600 grit paper. You will need to sand a few inches onto the old surface or to the nearest edge to prep a blend area for when you buff. Spray a few coats of clear, first only over the repair and working your way onto the surface you prepped a little farther each coat . The last coat or two should be cut with about 20%-40% reducer depending on solids content and taken just to the edge of the area you prepped. Let it cure as needed and you can start sanding out at 800 to prep for buffing. Never tried KTM for this but have used different automotive type clears over the years and all worked pretty well.
Author:  Freeman [ Fri Sep 30, 2016 10:33 am ]
Post subject:  Re: How do you deal with repairs to catalyzed poly finish?
Thanks Brian, that is very helpful. When you say "spray a few coats of clear" I assume you mean nitro but I could try the KTM. Would a couple of coats of shellac or vinyl sealer help the clear to adhere do you think? Would it be worth considering just doing it in shellac?

They owner says he doesn't care what it looks like but I'd like to use this experience to learn from. It seems like this should be a common problem yet I can only find one reference on line to this kind of finish repair - Frank Ford has a Taylor 12 string with a side crack that he comments about the UV poly - his approach is a couple of coats of shellac followed by nitro over that.
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