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Question about using a black light to inspect finishes

I've been using a hand held black light to inspect finishes on vintage instruments for awhile, it's a pretty amazing tool. For anyone not familiar, older finish tends to show up as a sort of olive drab color, and newer finish touch ups or refinished areas, along with areas that have play wear, show up as dark purple. I was curious though, does anyone know what it is that makes lacquer finishes of different ages show up differently under the black light?

I was especially confused after recently finishing a repair on a late 60's D-35; the guitar in question had had the original owners initials and social security number scratched deeply into the back of the headstock (blows my mind that he actually sold the guitar with his social engraved on the back of it). The new owner wanted this removed. I ended up having to sand down the back of the headstock until I had removed about .010" of thickness, and then I did a color match and refinish of the back of the peghead. There was also a headstock break that had been fixed about twenty years ago that the owner wanted hidden; I filled in the voids in the crack on the side of the headstock with epoxy, leveled it off, lightly sanded it back, and then gave it a few overspray coats. So when I was done, the back of the headstock was completely refinished, the sides were oversprayed, but with original finish and color underneath, and everything else was pretty much original, aside from a little bit of overspray down the neck to blend. Under the black light, however, only the back of the peghead showed up purple. The side where I had oversprayed showed no indication of having been messed with at all. My original theory was that maybe the olive drab color came from surface wear/contamination of the original finish, but that must not be the case, as I did sand back the sides of the headstock before overspraying.

For what it's worth, my color match and touch up on the back of the peghead was pretty spot on; in direct light, it was close to indistinguishable from the original finish, but under the black light, it became glaringly obvious that it wasn't original.


Does anyone have any insight on what causes different finishes to react differently to the black light?

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