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I think you need to use a dewaxed shellac sealer BEFORE using epoxy as a filler.

Like Woodie G, I suspect the dark spots are sand through areas through the epoxy. I'm thinking that the vivid color over most of the back may be due to the epoxy (which is the reason why some of us like to leave a thin epoxy coat over the whole surface) and the less vivid areas may lack a sufficient coat of epoxy.

I may have missed it, but what epoxy did you use. How did you prepare the wash coat? What did you use to dilute the epoxy and at what ratio? After you applied the wash coat, did you do any sanding or scraping of the wash coat? I'm wondering if for some reason the wash coat wasn't effective in fixing the sand throughs.

I'm wary of the naptha test to see if an area of sand through through the epoxy will match after the finish is on. I had sand through of the epoxy on a mahogany neck. When I wiped it with naptha, the sand through area matched up exactly with the surrounding areas. So, I went ahead and applied the finish and the sand through ended up being clearly visible as a different color. So, I no longer use that test. I sanded off the finish and re-did the epoxy and problem solved.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Assuming you have some "offcuts" of purple heart I would suggest you do some tests on scrap, to try to replicate the problem and also to find the solution. It may keep you from multiple sandings and ruining the almost finished guitar.
Also try using stripper on your test pieces (and bindings) to see if that may be a better way to remove the existing finish on the guitar. With Nitro, I've found it easier and less problematic to strip with lacquer thinner than sanding (although still messy). Methylene chloride is one of the few that stands a chance of stripping epoxies and polyesters.
If it is the EM6000 that is the problem you may be able to strip off only the top coat, reapply the epoxy to the sand through areas, give a light overall sanding to the epoxy and recoat with a different topcoat.
I've built a few instruments with purpleheart that held their color well, and as I mentioned before a dresser that was a purple heartbreaker - one part of the top turned brown almost immediately. The client accepted it - he had supplied the wood and was familiar with the risks of this happening.
Still-

Please DO NOT use shellac as a sealer under epoxy... and if you've already used it as a wash coat, sand to bare wood and hope for the best. Uniform color is achieved by ELIMINATING surface contaminates.

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For the times they are a changin'

- Bob Dylan

Update


So the most recent update.
I removed the finish. For the 2nd time:) yay.
Removed epoxy down to wood.
I knew I was risking issues if I had to sand this down again, hitting bindings and such… so… I simply sealed w 10 thin wash coats of shellac (probably overkill I know) then applied the Em6000.

Most of the pores are filled. I have a really nice sprayed finish with no orange peel yay.
3 more days of curing … it’s been done 3 as is.

Lessons learned


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Looks great! Congrats on ending the frustration.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Gah… I know right. For me as a hobby builder finishing is suuuuch a stress because I can get every joint right every measurement perfect and make every mitre seamless but I know finishing is around the corner ha. At which point the entire guitar has the possibility of going from lovely to meh


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Way to go. Congratulations on being persistent and getting it done!

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Steve Smith
"Music is what feelings sound like"

Steve, that’s definitely something I’ve struggled with. I’m to the point where WHEN I screw up … I take a step back.. remain calm and simply recognize it as another step to do (fixing the mistake) before the guitar is done
Needing it done on a time schedule adds to the stress though ha


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Looks great! I'm glad you got it sorted out. So I guess the dark spots weren't sand through or it would all be dark now. Do you think those spots were excess epoxy? It sure would be nice to know.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Well I feel like the dark spots were sand through…
I actually removed ALL the pore filler and out shellac wash coats down… I’m still not super sure…. But I’ll definitely be paying closer attention next time.


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Way to stick with it! Finishing has always been the steepest part of the luthier learning curve. The trick is remembering to get back on the horse!

Looks great!

Best, M

I don't quite get it. How does sanding further remedy the sand through? Why was sand through darker under finish but by removing all the epoxy the dark spots are gone?

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Well what I mean is I completely removed the epoxy . There’s no epoxy on the instrument now. My feeling was that, for whatever reason the epoxy was the issue… not sure if maybe it is because the epoxy maybe allowed the water based top coat through in this areas

At any rate there’s no epoxy and no dark spots. Therefore the epoxy, in some capacity, was the culprit


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