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What are Medullary rays...
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Author:  John Elshaw [ Sun Dec 17, 2006 2:59 am ]
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I'm just about complete with a spruce top classical and have a question. I sanded the top starting at 180, then 220, 400, then 600. The top was smooth as glass ready to finish. I put a wash coat of shellac on and all of the sudden it looks like I sanded with 80 grit opposite the grain!!!   

I've heard of Medullary rays but am not sure what they are. Is it similar to flame or a figured type pattern it gives to the top? The "ugly, poor sanded" pattern in the top really looks like something natural, and it's even bookmatched which tells me it's not my sanding. Does anybody have any idea what this could be? I'll try and get pics later but I'm not sure how well it will come out in a photo.

Thanks!

John
Author:  Bobc [ Sun Dec 17, 2006 3:26 am ]
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John that is also know as silking and will occur when the wood is well quartered. Actually it is sought after by many builders. Here's a link describing medullary rays in oak.

http://desertcraftsmen.com/Mystique.shtmlBobc39068.478599537
Author:  crazymanmichael [ Sun Dec 17, 2006 4:03 am ]
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did you raise the grain and do a final sand before applying the wash coat of shellac?

it sounds as if the wash coat of shellac raised the grain and made it stiff. very lightly sand it off and it will be smooth as before.

make sure that you do your final sand with the grain and not across it.
Author:  Rod True [ Sun Dec 17, 2006 4:22 am ]
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Here's a good picture of Medullary rays

Author:  TonyKarol [ Sun Dec 17, 2006 4:43 am ]
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Wood only needs to be sanded to 220-280 prior to finish. Nitro needs no sanding betwen coats, unless its only to level it - there is no mechanical bond at all between coats, each additional spray coat simply melts the previous amount of finish thru its solvent base, and then dries and cures into a single thicker coat.

Other finishes, like poly (urethane or ester) benefit from light sanding - they do not generally cross link or melt previous coats - that is why if you sand thru one to another you get usually witness lines, unless you again recoat.
Author:  Mattia Valente [ Sun Dec 17, 2006 5:02 am ]
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I sand to 320-400 grit on tops, 220 on backs/sides. If I'm oil finishing, I'll go to 1200.

Here's a shot of planed, unfinished wood, with some medullary rays (grade AAA top from Rivolta, Euro Spruce)

Author:  John Elshaw [ Sun Dec 17, 2006 10:25 am ]
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Thanks fellas, my guitar looks a lot like the pics of Mattia and Todds guitars. I guess it is silking, the only thing that looks really weird is while it's still unfinished, it almost looks splotchy like darker and lighter wood because of the different reflections. We'll see what it looks like when I'm done.

Hesh--the only reason I go to 600 grit is because when I French Polish, it seems the glassier finish comes much more quickly when I start with a smoother surface. You could be right though and that might all just be in my head!

Rod--you're rays look much more even and clean. I hope mine improve under the finish.

Cheers!

John
Author:  Alan Carruth [ Sun Dec 17, 2006 2:02 pm ]
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Since they run exactly perpendicular to the annual ring lines, you only see medullary rays properly when the wood is very close to perfectly quartered. I beleive it's the rays that give perfectly quartered wod it's higher cross grain stiffness and lower shrinkage, when compared to wood cut tangentially. So, if you cut two tops from the same tree, and one shows more ray patterning, it will generally give you some 'free' stiffness.
Author:  Mike Collins [ Mon Dec 18, 2006 7:40 am ]
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There what Medullaries shoot at ya!

The above examples and explanations are great!

Mike Collins
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