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.
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.