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Here's a phenomenon I can't quite figure out and I've seen it with oil based polyurethane as well. I have had to go back and refinish some sand through in a couple of spots on some very dark EIR (though I've seen it on Mahog as well). In a couple of places I sanded through the KTM to the epoxy with no change in color but on a couple of other places I decided to sand back to the wood to repair some small imperfections.

In those spots where I sanded back to the wood I applied the same epoxy in the same manner as the rest of the guitar then applied the same KTM-9 in the same amount but the color changes under these spots to a lighter tone.

It is pretty consistent and I think a pretty well known phenonmenon but I don't understand the science - why would the same finish materials end up lighter upon a second finish application which seems to be the same as the first?

Larry

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The odds are that you saned through the finish but not all the way through the epoxy on the ones that did not change tint.. However if you reapplyed epoxy and finish then I am not sure I get it providing you allowed cure time on the epoxy... This is strange. but then I have never applyed KTM9 directly over epxoy either I always seal the epoxy with a top coat of shellac.

I know this is bound to come up. So!! I put the shellac over the epoxy. Never under the epoxy. Epoxy will not adhear well to shellac. Shellac will adhear well to lightly sanded epoxy just covering by bases in advance

It could be that you exposed wood that had not darkened due to light....

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[QUOTE=Don Williams] It could be that you exposed wood that had not darkened due to light....[/QUOTE]

Yeah I thought about that Don but don't remember enough time between sanding and the first coat of epoxy to have allowed the wood to darken from the light. Hmmm...

Either way it is quite visible and quite annoying. Perhaps I'll try exposing the bare areas to light? Are we talking UV sunlight? Which I doubt the wood was exposed to - or just light? Or the air? How much time will do the trick?

larry

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Some woods can oxidize fairly quickly and it doesn't have to be direct exposure to the sun.

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Hech!! sand down some Cocobolo out here in the H2S rich atmosphire of West Texas and watch it change right before your eyes

Changes in refraction? I've run into similar situations where the repair shows only when the light hits at certain angles (not reflection). In one case, it was stain, shellac, filler, shellac and lacquer in that order over a black walnut neck. The color and lacquer blended very well. Looked perfect even with strong reflections on the lacquer but when the light hit at a certain angle, the color looked off. I suspect it was due to not being able to go deep enough to get all the finish out of the wood before putting on the new finish. Instead of the original stain, shellac, filler and lacquer, I probably had something like stain, shellac, stain, shellac, filler shellac, lacquer,when I was done, with some of the original shellac and stain sitting under the new finish. Wonder if you have something similar going on here.

Maybe the epoxy you're re-applying is not having the same wetting effect as the original epoxy, since it is in a sense epoxy over epoxy instead of epoxy over wood, assuming that some of the epoxy remained deeper in the wood after sanding?burbank38755.7143865741

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[QUOTE=burbank] Changes in refraction?

Not refraction from what I can see.
[/quote]

[quote]
Maybe the epoxy you're re-applying is not having the same wetting effect as the original epoxy, since it is in a sense epoxy over epoxy instead of epoxy over wood, assuming that some of the epoxy remained deeper in the wood after sanding?[/QUOTE]

Yeah that's my suspicion though I used the same technique in other places with no color change. Maybe I went deeper in this area. I'll sand a LOT of the back down and start over and see what I can see.

Larry

Thanks for the replies and ideas.

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Here's my theory - After I sanded the area to include some bare wood I then cleaned the area really well with denatured alcohol. I think that the alcohol dragged some KTM-9 into the bare wood and sealed it just enough to keep the epoxy from 'wetting' the area as much as other areas. this assumes that water based stuff like KTM9 won't wet as well as epoxy or oil base finishes.

I sanded down to bare wood with no alcohol and it seems to be as dark as the rest of the wood.

What do you think?

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[QUOTE=MichaelP]
I put the shellac over the epoxy. Never under the epoxy. Epoxy will not adhear well to shellac. [/QUOTE]

So if I use shellac to seal the wood (so I won't get staining when gluing the bindings)I should sand it off before applying the epoxy pore filler?

Kurt

[QUOTE=klhoush] [QUOTE=MichaelP]
I put the shellac over the epoxy. Never under the epoxy. Epoxy will not adhear well to shellac. [/QUOTE]

So if I use shellac to seal the wood (so I won't get staining when gluing the bindings)I should sand it off before applying the epoxy pore filler?

Kurt[/QUOTE]

with out question YES! Actually I would never put shellac on ever before epoxy If I have a binding that may stain and I am going to use zpoxy to fill with. Instead of sealing with shellac to prevent the stain I wil mak a 60% Zpoxy 40% alcohol mix and use it as I would shellac. The problem is that the epoxy will not bond well at all to shellac. The epoxy will want to peel off. more prevlent on large flat areas but non the less I would not use it over shellac.