Official Luthiers Forum!

This seems odd. I’ve done some sanding with 180/220 but literally just for the past 10-15 minutes if that and my braces are showing. The strange thing is that the top is still pretty thick-0.110” or so.

Getting rid of the red coco stain around the rosette is not proving affective. Seal with shellack and give another go?



Here’s the back for anyone that’s curious.



Sent from my iPhone using Tapatalk

Did you have some shellac or finish on it before sanding? My assumption would be that the braces provided more resistance to the sander and more finish was removed over them.

It may be too much downward pressure on the sanding block. With or without finish on the top,
you'll get that effect on spruce but not on harder woods

I was hand sanding. Learned my lesson with the ROS one too many times!

How much sanding had you done before putting the braces on? I do almost all of the sanding of the top after the rosette is in, but before putting the braces on while it's still flat. Very little if any sanding after the braces are on.

Have you tried using a scraper to remove the coco dust?

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

One likely cause is a humidity change from when the top was braced to when you did the sanding, especially if the RH is lower now. What happens is the top tries to shrink but the braces are holding firm. That will result in top retreating slightly around the braces.

I would use a foam pad while sanding and bring it back to nearly flush, or better yet, rehumidify the body and then procede.

Also, that is some beautiful ziricote and rosette/ It is going to be a fine guitar!!!

_________________
Joe Beaver
Maker of Sawdust

As for the coco dust - scrape with a fine burr from the spruce ONTO the rosette so any coco shavings travel further away from the spruce, then avoid sanding at the coco/spruce joint.

Ed

Why are you sanding this now? You don't sand the instrument till the box is together and bound. Rough sand the rosette flush and leave it at that. Sanding now just takes thickness off the top, and after it's bound you will just have to sand it again.

Still not sanded except for rough sanding rosette...


The telegraphing looks like humidity problem to me...

I usually sand to 220 with the drum sander after the rosette is installed. However, this is my first cocobolo rosette and I did not anticipate the coco oil bleeding into the top. So I stopped and figured I'd just thickness the other side before bracing and finish sanding the top by hand. That's where I'm at now.

Thanks, it's actually Cocobolo but gorgeous stuff either way. You don't find coco like that much any more these days.

Good point. I was trying to sand the discoloration of the coco oil out of the top around the rosette, it's already flush just discolored. Scraping lightly didn't do the trick either. I suppose I'll bind it, cover the top and move on. I don't think it's humidity. The shop is kept between 40-50% unless the power is out depending on how much it rains and how much the dehumidifier can actually remove. There are 5 other closed boxes in this batch and none of the others are having this problem. I may just be pressing too hard as mentioned earlier, I am a bit heavy handed.

I didn't seal it. I usually do that after binding & before masking the top with poster board.

What kind of spruce is this?
Sure is "dirty". If that's what those AGF rosettes do, glad I never used them. I used to sand BWB purfling and it all came out...
Could be mushy spruce too, Engelmann is prone to making a mess. Maybe that's why I liked red so much, no blow outs when routing, didn't get that dirty, never telegraphed and used a RO all the time. Long as you go through the grits and raise the grain, you are fine. Nah, was headroom and tonal qualities.

If you are scraping rather than sanding - then you shouldn't have too much trouble with various woods staining the top. However, any type of modern sanding with sandpaper is always problematic. The solution is dust collection when you are sanding. Also, when next to the rosette of binding - move from spruce to the hardwood to prevent the dust from bedding itself into your top.

I highly recommend getting a vacuum sanding block - I got mine from Mirka years ago and it is still going strong. I use a Dynabrade for sanding the top and back after binding but finish up with the Mirka at 220 grit. My binding makes a mess during level sanding and my tops always look filthy - but they clean up very easily. Remember, you don't want to remove too much material during this phase as it can be difficult to maintain consistency from guitar to guitar if you aren't sure how thick your tops are ending up.

This is actually a really tight grain red spruce top. I made the rosette myself out of the scraps from the Coco backs and sides. I figured that the coco had sat long enough (5 years) for the oils to "crystalize" as another builder told me would happen, but apparently I didn't wait long enough. That stuff was the oiliest wood I've ever handled. Most of the HHG bracing on the back failed so I had to undo everything and re-glue with Gorilla, which I hate using. It makes such a mess.

The other set of Coco I built with in this batch was fine. It took HHG and all. Same type of rosette and same specs for the top too, red spruce from the same vendor. That coco was much older though and it didn't seem to be nearly as much of a problem when leveling the rosette.

I was able to mostly clean it up sanding from spruce to hardwood with a sanding block as you mention, but it's not perfect. I'll worry about it when it comes time to finish sand. I'm not too worried about the thickness right now, I left the top pretty thick and I've got one of these on order for final thickness setup now that the braces and everything are installed:

https://www.stewmac.com/Luthier_Tools/Tools_by_Job/Tools_for_Measuring/MAG-ic_Probe_BLE_Thickness_Gauge.html

I'm not familiar with Mirca or Dynabrade products. When I use a ROS I'm using a Bosch with a universal adapter (Woodcraft) hooked to my shop vac. The staining happened running the top through the drum sander for leveling as I normally do. I may give the Bosch a shot when it comes time to finish sand depending on how thick the top is and how it feels, but I'm leaning toward just doing it by hand. Does that Mirka sanding block have a 1" port? If so it would work with my current setup which would be perfect. Either way thanks for the tip.

Are you blowing off the sanding dust with compressed air? That will usually remove rosewood sawdust but I am not sure about cocobolo.

I use Mirka Abranet with a Bosch RO hooked into a 3hp dust collection system for the finsh sanding, 120/180/220. Haven't had the problems you're experiencing. After building up the finish(nitro), 400/600/800/1000 Mirka, then on to Micromesh or buffing. Haven't used cocobolo though.

Mighty small soundhole... I thought these got cut out before the bracing was installed.

_________________
Peter Havriluk

You can if you want but I don't. I like to keep dust and stuff out of the box so I cut them by hand just before gluing the fretboard on.