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Calling any folks versed on repairing old nitro finishes!

After replacing crumbling binding on a 60s Gretsch, I oversprayed new black lacquer around the perimeter, scraped the binding flat, did a handful of black drop fills, then oversprayed clear nitro over the new lacquer areas, feathering into the old lacquer.

When I was leveling the new lacquer I started to see witness lines between the new and old layers, and now as I'm buffing the finish they are getting more obvious. I suspect that the new lacquer did not burn in adequately with the old. I used slightly thinned Behlen, no retarder.

What are my options at this point? Can I overspray thinner or retarder and get the new coats to burn it? I have 4-5 coats on, sprayed a couple weeks ago.

Thanks!

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I had something similar to this recently, filling in lacquer over a crack repair in a guitar top. Tried the thinner and retarder approach, and well thinned coats with some retarder. No luck. I had to scrape down to the existing lacquer carefully (which was easy since it just flaked off once I got close), then rough the area with 320, and pre soften with with acetone to get new finish to bite and melt in. I originally applied lacquer thinner to pre soften the area - apparently that didnt do it. This was a 78ish Larrivee. I guess with that much time to dry the lacquer becomes quite hard and requires something more aggressive to soften it. I also wonder if guitar polishes with wax helped to form a barrier in some way - I had cleaned the area thoroughly beforehand though. Before I took acetone to the area, I tested it on a hidden area (I had the neck off too), and it took a good wet drop sitting there for a good 30 seconds to do anything to the finish, and it still wasnt visible until I scraped the spot with my fingernail (my nail could not scrape an adjacent untreated spot) - my lacquer thinner didnt touch it at all.

Good thing for me the area was very small, so starting over wasnt too painful. Your situation is not so fortunate. I think if you were to just spray thinner/retarder, you would have to spray enough to penetrate both layers of finish, otherwise the witness line will come back - and that sounds like a mess. I think your simplest option is to feather your new lacquer into the old with 320 grit paper, pre soften it with the appropriate solvent, then do some more topcoats as needed to feather into the old stuff again. Id test various thinners on a hidden area to see what does the best job of softening it. Of course if you go through those while levelling and buffing, the witness line will show up again, and the adhesion of the new lacquer where you didn't sand and pre soften may be suspect regardless - it could start flaking in a year, or ten years. Your other option is to strip back your work and start it over - not so fun :(.

I really hope someone else here has a better option for you...

Edit: I should add that Im not well versed in old nitro, but I felt my experience was similar to yours enough that it could be of use to you.
Fun, isn't it. ;)

Hard to eliminate these with touchups on old lacquer. A quick and easy trick that may help though -

When I can't get a full almagamation of old to new, and witness lines keep coming back in final buffing, I will often do a light misting of retarder (butyl cellosolve / ethylene glycol monobutyl ether) on the seams with no follow up sanding. Finish level sand as though you are about to rub out, very high grit paper (2000-2500), then go over with a very light misting of retarder.

A day or two later, I will rub out with a compound like Meguiars #2 or Jescar polishing compound, rather than take it to the buffing wheel. With this approach I can often get the lines to finally disappear, but the blending really only occurs at that tiny feathered layer where they meet. Sand back or buff out too aggressively, and the lines can come back.

Of course I will usually hit the original surface with retarder before spraying the touchup, and use a bit of retarder in the lacquer mix throughout the touchup process as well. In a pinch though, this top misting of retarder with no sanding or buffing afterward can help quite often.
Thanks to you both. I picked up some retarder today and I will give that a shot first.

To follow up ... the retarder overspray worked out with a bit of trial and error. The retarder melted into the new lacquer more easily that the old, and so it tended to gloss out the new part  and made it look too new next to the old stuff. So, it took a few rounds of sanding/spraying/polishing to get a good even result with out the lines reappearing, and also getting the right gloss level. 

I found this thread using Google. I have been spraying nitro and doing touch ups for years and have never witnessed a witness line. Until a few days ago. On my Les Paul I was touching up the area over my filled screw hole. I had used my airbrush but apparently didn't thin it enough. I first tried using straight retarder on a cotton ball and as a guinea pig used the back of an acoustic which I had touched up last year. It did reduce the line but under shop light you can still see it. So I airbrushed straight retarder and the only evidence now is a raised area where the new lacquer had swelled from being rewetted. I will buff this in a few days. I noticed that looking at the witness line with a magnifying glass it is actually a line made by many tiny holes. Rewetting it apparently fills these holes.

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