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i've got a cedar/rosewood OM at the finishing stage and it's driving me nuts. twice now i've had about 4 coats of finish on the cedar top and decided to sand it back to bare wood because i see blotches.

the blotches are around the edges and at the x. i'm sure it's the rigidity of the bracing and kerfing that's telegraphing through. i just can't figure out how to lick the problem. the top looked great before i began, but it must be my sanding technique or something about cedar that's vexing me.

i'm sanding with norton papers and using a fairly dense foam block as a backer. when i knocked off the finish i used 150, followed by 180 and 220. i think that's fine enough. the grain looks great. i just can't seem to get even color across the top.

oh, i'm using ktm, but don't think that's a factor. i'm sure the blotches i'm seeing would show up with any finish.
i've used the same finishing technique with sitka and engleman tops and always seen even color on them.
what am i doing wrong, or what don't i know about finishing cedar??
thanks for any advice.
phil

Phil, i've only built with cedar, granted that is only 4 guitars now, but I have had good success with the evenness of the wood once sanded. I do sand up to 400 though on the cedar, that sure makes a nice smooth surface for the sealer. What are you using for the sealer before the KTM?

I don't really know that much about finishing either, so all I can tell you is that I sand the top to a higher grit than the rest of the guitar.

Here are a couple of pics of the last guitar I built with nitro as the finish

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how thick is your top? wrc usually needs to be left a bit thicker than spruce.

I'm just guessing here, but....

If you used that same foam block while sanding the bare wood, you might have inadvertantly sanded the more flexible areas differently than the more rigid areas, creating a shadowing effect with the wood, similar to seeing runout color change. When sanding a top especially, you want a hard, solid block so that your sanding is even.
Michael also mentioned something that concerned me as well, which is the thickness of your top. I know some schools of thought are to get a top really thin, while others are to keep a cedar top a bit on the thick side. I personally lean toward the thicker side, and wonder if your top is a tad thin, causing the bracing and linings to telegraph through.
Also, if you didn't seal the wood prior to finishing with ktm, you're wetting the wood and that can cause issues of it's own. It's always a good idea to seal the wood with either a sanding sealer or something else prior to spraying a waterborne.
Hope that helps.

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I'm working on a red cedar top now and even at 0.130", it seems too thin, it flexes a great deal across the grain so, not to hijack, what would be the best thickness for that wood please? TIA

Serge

serge there is no right answer ... I hav used cedar tops that were 150, and otheres that went down to 110 .. its all in the stiffness of the piece .. BTW,the one that was 150 was a little quiet, but what a tone - very rich and complex, and tonnes of sustain. EIrw B&S.

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OOps .. forgot to post on the origianl topic ... after using the RO sander to get things cleaned up (100 and 250/280), I sand with 280 on a piece of flat hardwood or birch ply, whatever is kicking around on the bench, no cushioning, especially around the binding edges - the RO will tend to dish the top here, particularly betweent the rosette and waist. Sand flat with the board, then a quick touch again with the RO if any binding dust is on the top. The RO (250 grit) has dust collection and leaves the top clean.

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Tony -

Out of curiousity - if we use "a piece of flat hardwood or birch ply" as a sanding board with "no cushioning" how does that work with our radius on the top? I do understand the intended purpose of the flat hard surface - no hollowing out soft or pliable areas. But couldn't we be potentially creating facets while sanding? Granted they would be small - but I would imagine that the hollowed out areas would also be small - and probably not that noticable until you put that nice gloss finish on it.

[QUOTE=TonyKarol] serge there is no right answer ... I hav used cedar tops that were 150, and otheres that went down to 110 .. its all in the stiffness of the piece .. BTW,the one that was 150 was a little quiet, but what a tone - very rich and complex, and tonnes of sustain. EIrw B&S.[/QUOTE]

Thanks coach, my top isn't stiff across but has good stiffness along the grain, i'll probably have to make my braces tall and thin to get more sustain out of the closed up box. Thanks

Sorry again for the hijack

guys,
thanks for all the advice.
my hunch is that all of you are correct. i think my top is a bit thin for cedar (though i made sure to leave the bracing a bit taller than i have before). i'm going to go with the harder block and see what happens. i might also try sealing, though i haven't done that with the last 4 guitar and still got good results. perhaps the cedar requires a different treatment than spruce there.
so thanks for all the advice. i'll let you know how it turns out.
rod, by the way - beautiful wood and guitar. thanks for showing that off and giving me good reason to keep sanding until i get it looking right.
phil

Hesh is right, plus you are using too much down pressure. Let the paper do the work, if you need to push down, change paper! Do what the high end auto finishers do, use a stir stick wrapped with paper, if it bends you are using too much pressure. It will follow the curves easily. Sand to 600 minimum and save yourself a lot of effort later on. You don't want to sand wood, you already did that, just level and smooth the finish.

Dean

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Dean

I for one dont bother to level sand until I have 4-5 coats on - I dont see the point in trying to level after each coat, with nitro you have only added another thou max per coat. I did experience what you are talking about though a ways back - you have sanded thru to the wood on cedar, and for hwatever reason, it doesnt come back to the same colour after spraying more ever.

i also discoverd that this particular piece of cedar I am just finishing doesnt like Zpoxy. After sanding nice and smooth, the wipe on coat across the top looked very blotchy, even though it was very thin. So off it came, touch sanded the rest of the Zpoxy covered cocobolo body, and hit it with the nitro .. looks good now, nitro right on the bare cedar.

Hesh .. not sure what finish you are using, but nitro does not bond mechanically betwen coats, looking for adhesion - it simply melts the previous coats, then when dry it is one thicker single unit of finish.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"