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Specs In Finish


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After staining with stewmacs stain, spraying 5 mist coats, spraying 6 build coats, and letting it dry for 16 hours I started to level sand. The back (mahogany, stained dark red mahogany) went perfectly and looks amazing. But when I sanded the top I noticed all these little white specs in the finish. I can't get them out, I've tried sanding, wiping with water, and wiping with naptha. They just won't go away :D They do -not- appear on the headstock which is the exact same maple stained the same way. The finish is two part poly.

It looks like they are sanding dust laying on top of the finish. But for some reason they won't go away. Even after trying to sand them off.

In this pic you can see what I'm talking about. pic

Here is one taken through a microscope. Notice the particle (all of them are) is exactly round B)

Here is another shot from the microscope. This one was a test piece that was stained green. Notice again how the particles are round.

Any ideas? I can't seem to figure out exatly what caused this and why I can't get them to go away. :D

Edited by Godin SD
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They could be moisture bubbles that were gassing off the wood when you shot your finish and migrated up thru the finish, but couldn't make it all the way out, thus trapping them in the middle of the finish.

I get them sometimes no matter how much I acclimate the body to the surrounding environment before shooting, so I only ever shoot 2 coats when doing the first initial coats. Nice thing about nitro tho is I can gently sand them out and the next coats just melt everything together again, something I don't think would happen with poly.

Either that, or maybe they came in with the poly, was it a humid day, and do you have moisture traps in your rig?

One way or the other, I think they're moisture pockets of some description.

Have you tried shooting another coat over top? They might just go away with another coat, especially if they were moisture pockets migrating up from the surface of the wood, once you have the initial coats on, that problem is now gone, the wood is sealed.

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Could they be pinholes in the clearcoats, which have become filled with sanding slurry when you levelled/buffed the top?

That was what I thought it was. But then why can't I seem to get them out?

And if they are pinholes like Drak said, how should I fix it and prevent this problem in the future?

Edited by Godin SD
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Could they be pinholes in the clearcoats, which have become filled with sanding slurry when you levelled/buffed the top?

That was what I thought it was. But then why can't I seem to get them out?

And if they are pinholes like Drak said, how should I fix it and prevent this problem in the future?

Not sure about 2 part poly but with automotive clears if you dont let each coat flash long enough or lay it on to thick you can get solvent pop. Solvent pop is caused by the trapped solvents in the previous layer gassing out to the surface and getting stuck as an air bubble. What you have looks like solvent pop to me and can only be fixed by sanding until they are gone.

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[Not sure about 2 part poly but with automotive clears ...

2 part poly IS automotive clear

Well if the 2 part poly is indeed auto clear I would say you have solvent pop. To much to fast is generally the problem. I use Dupont 7900s high build clear and it is very forgiving and solvent pop is very rare. I generally wait 15/20 minutes between coats of clear. 1 tack coat and 3 wet coats is my routine. solvent pop is not as much an issue with the 2 part clears but if you are adding reducer also it is much more an issue.

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