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ChuckBarnett
11-14-2018, 05:07 AM
Yet another rookie question...

I'm at a hundred and fifty grit sandpaper using a block made of hard rubber, going with the grain. I'm seeing what seems to be spots that are sanding more quickly where there is bracing, etc as backing. Am I to assume that eventually this whole thing will be relatively level? Or should I be doing this some other way?

As always, so grateful for this forum!

mikeyb2
11-14-2018, 06:24 AM
Hi Chuck, this is likely a humidity issue. Either the humidity wasn't ideal when you glued up the braces, or it's not ideal now. I would check it before you go any further with sanding.
Mike

sequoia
11-14-2018, 06:31 AM
I see what you mean. This is called "telegraphing"... Obviously the top is not level and is cupped slightly around the bracing. This probably happened when you glued your bracing down. It is important to glue the bracing in against a hard surface; a flat piece of melamine for a flat top or a radioused dish if the top is radioused. That being said, I would not continue with a sanding block because the wood over the braces is going to end up thinner than adjoining areas. Not a good thing probably. If it was me, I would sand with my hand the areas that are not getting sanded and increase my grits until 220 sanding evenly everywhere. I sand out to 320 and I use circular motions with the final sand out. Use a powerful light to reflect off the surface to detect deeper scratches from previous coarser grits. Hope that helps.

ChuckBarnett
11-14-2018, 06:46 AM
Makes sense to sand by hand at this point. I did glue in a radius dish.

ChuckBarnett
11-14-2018, 06:51 AM
I'm in western Washington state and although a few others in this area seem okay with humidity I glued at below 45% (per the setting on the room dehumidifier). And I'm sanding at lower than that.
Could another consideration be that it's taken me a long time to get to this point and does the would actually bend around the hard backing over time without finish??

ProfChris
11-14-2018, 08:31 AM
Wood moves (and quite a lot when thin) even with small humidity changes.

I'd hand sand using a small block - I use either a pencil eraser or a wine cork cut in half lengthways. This rolls up and down the inevitable small undulations, so you don't get the problem unless your telegraphing is severe. On soprano ukes at least, there's no real time saving by machine sanding once you're up to 150 grit or so - 2 mins per grit for the top or back, max.

ChuckBarnett
11-14-2018, 08:48 AM
Pencil eraser as in the photo?

mikeyb2
11-14-2018, 10:07 AM
I'm in western Washington state and although a few others in this area seem okay with humidity I glued at below 45% (per the setting on the room dehumidifier). And I'm sanding at lower than that.
Could another consideration be that it's taken me a long time to get to this point and does the would actually bend around the hard backing over time without finish??

if you're sanding below 45%, what exactly is the humidity? I wouldn't rely entirely on the dehumidifier setting and get yourself a another reference hygrometer to check. It's possible your humidity is too low, hence the telegraphing, and the top might normalise when the humidity rises. I had this exact same problem on my second build, but it straightened out when the humidity went up. As I said earlier Chuck, don't do any more sanding until you know the humidity is right.

greenscoe
11-14-2018, 08:45 PM
Comment removed.

Titchtheclown
11-14-2018, 10:43 PM
I think what you are seeing is not cupping or permanent movement or anything to do with humidity or wood whispering mumbo-jumbo magical talk but simple flexibility. The areas supported by the braces is stiffer than the things areas between so it can not flex away from your sanding block. The areas in between react to the pressure of your sanding block by flexing away. The pressure =sanding is therefore optimised to the areas above the braces.
The solution is the same. Small sanding blocks that don't span the area between the braces and light pressure.

mikeyb2
11-15-2018, 03:19 AM
I have to disagree with what Ian says above. What he says may be true to a small degree, but the main problem is humidity. Glue braces to your top in the wrong humidity and the results can be seen quite clearly as telegraphing of the braces. This can be noticed before it's been near any sandpaper.

Jardin
11-15-2018, 04:02 AM
So I am am going to through in my 2 cents for whatever that is worth here.....& it goes towards both of your current threads (this one and the one about buying an accurate hygrometer)....

Do you know what the Relative Humidity (RH) was when you braced & did your glue ups? (using the hygrometer you have now)

That is where you want to work on this particular instrument.....Not where a new hygrometer is saying it should be.....if your current equipment reads say 45%RH & that is where you glued your braces on & you get a nice accurate one that reads 38% while that one is at 45% you want to stick with the same RH you started with......not the new baseline as that is not where you glued up

Yes, there is an ideal RH when we build & ship instruments.....yes there are ideal RH ranges for your local area....but the fact is that you glued these pieces of wood together at ___% and that is where you want to finish this instrument (in my opinion).....Then you can go ahead with your new tech & establish your new baseline for accurate RH on the next instrument(s).....

The take home is not what the actual/accurate RH is...it is what RH did you glue at......I hope that this helps & makes sense....Hang In There......You Got This!