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So, with 5 coats of ktm9 brushed on my spruce top, I sanded through to wood in a couple small spots while level sanding. D'oh! The five coats were put on three weeks ago so the KTM9 has had a few weeks to cure. The rest of the top has been sanded up to 800 and is now quite smooth.

Can I apply more ktm9 on the cured finish thats already there and will it bond correctly? Should I scuff the top somehow for better adhesion? Do I cover just the spots or the whole top? What would you recommend?

Thanks.

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Keith Lally
http://krl-guitar-building.blogspot.com/

Keith -

I've done the same thing with KTM 9 - except on a neck repair rather than on the top. It burns in very well. Assuming you've got nothing else between the KTM9 and the top (shellac?) I'd just scuff sand all over the top with 400, brush on a couple of coats all over, then level again, once it's cured (3 weeks?), and buff.

Another relevant question is how are you going to avoid burning through again? You might want to revisit your approach to leveling. I've never brushed KTM 9 - always sprayed it on, but my approach seems to work without burning through. It is as follows:
Wet sand with a little dish detergent in some warm water and a sanding block with a rubber buffer between the sandpaper and the block, to let it conform to the slight arch of my top.
Start with 400, then 800, 1000, 1200, finally 2000. When I spray KTM 9 I tend to get some micro- orange peel, so I end up with little "sparkles" where the depressions from the orange peel haven't yet been fully leveled. For me this is indication that I can go back to 400. (very carefully). Once I finish with the 2000 I hand polish with an auto buffer and some medium polishing compound. Finished result won't be mistaken for a factory finish, but it's very nice.

Good luck!

Corky pretty much nailed it.... If you use any sealer underneath, be sure to re-seal the area or you will notice the patch. Good luck.