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How do you get the surface of your soundboard perfectly flat before finishing? I've tried scrapers, RO sander, sanding blocks (cork-backed and not) and it's always a challenge to get things level where the softwood meets a hardwood (especially around rosettes). For me, few things about instrument making are as infuriating as laying down a layer of finish and finding a ripple or wave in my soundboard.


I'm about ready to start prepping a guitar for finish and thought I'd check to see how others here go about this process. So, if you've mastered this step, I'd love to know what you do.

Thanks in advance,

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George :-)

Hardwood sanding block, no cork. The backer needs to be as stiff as possible to cut both hardwood rosette inlays and the softwood top at a uniform amount. Go slow, light pressure higher grit paper (180/220).


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I am sure the finishing experts like Brain H. will have comments --- but in my experience, sanding Spruce with the grain as "most always" recommended (there is a school of thought that indicates cross grain sanding is better for soft woods) tends to whittle out the soft grain leaving ridges. To prevent this I simply use a RO sander. If you think about it seems the RO is sort of working cross grain as it rotates. I finish up with 320 G

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Ken Cierp

http://www.kennethmichaelguitars.com/

At a guitar day seminar at ExoticWoods last year, Sergei de Jonge said that he sands the top with a hard block in a circular motion so that the hard lines sand at the same rate as the soft growth.
I've sanded only one top in this manner, and it is not showing any rippling or cross grain scratching, with one coat of finish left before level/buffing. With my very limited experience, I would suggest trying this on a sacrificial top or scrap first, and not on my say so.

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

No foam or soft backed sanders, especially no ROS. They will always dip the softwood around harder material.

As others have said, wood sanding block and scrapers. A drum sander with light passes followed by finish sanding with a wood block is perferred.

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Joe Beaver
Maker of Sawdust

As others have mentioned you want to use a hard sanding block. I personally do most of the sanding with a hard block (large block over most of the surface and smaller block around the rosette and waist) but will do a quick pass with the ROS sander between grits. I find that the ROS sander with vacuum dust removal is the best for picking the dark sanding dust our of the softwood top. Especially helpful when you have a guitar with a spruce top and dark binding like ebony. These are quick passes and I always keep the sander moving. I finish up sanding 220 with the hard block and then a very quick pass with 320 on the ROS. This method has worked really well for me when it comes to keeping everything flat and also keeping dark dust out of the light coloured wood. I really like my Festool ROS sander for this as it has a smaller orbital pattern than most standard ROS sanders.

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Josh House

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I would venture to say that all the higher production makers use RO sanders prior to finish application -- (check out the factory videos) I have not had a problem with ripples -- as mentioned that is why I do it that way. The pads on our sanders are very hard, maybe that is a key factor?

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Ken Cierp

http://www.kennethmichaelguitars.com/

I'll mention here that early on I was taught to dampen the surface with water to raise the grain etc. I discontinued this practice because it was doing more than just raising the fuzz which got sanded off anyway. For me its dry all the way.

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Ken Cierp

http://www.kennethmichaelguitars.com/

I think that is key. The original ROS sander I bought when I started building had a hard plastic pad. It worked great for levelling. When it wore out a few years later and I bought the same model (the updated version) they had replaced the hard plastic with a stiff foam. The stiff foam did not work as well as the hard plastic, and that is when keeping things level with the ROS sander became an issue for me.

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Josh House

Canadian Luthier Supply
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House Guitars - Custom Built Acoustic Instruments.
http://www.houseguitars.com

I guess I'm an outlier on this. Based on the number of people using a hard block with success, that approach obviously works, but I can't get my head around the idea of sanding a convex surface with a flat hard block. I do almost all of the sanding of the top and back while they are still flat before putting the braces on. I use the drum sander with 120 and follow with hand sanding with a hard block to 220. The hardwoods I've used in rosettes include rosewood, cocobolo, bubinga, ebony, and mahogany and there have been no problems with keeping the soft and hardwoods level. Once the braces are on, I do any needed final light sanding of the top and back with a firm foam block that conforms to the curved surfaces.

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

I should say I like RO Sanders and use then quite a bit, but they don't do well when there are hard and soft surfaces involved.

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Joe Beaver
Maker of Sawdust

Thanks, I appreciate the replies.

My drum sander does an excellent job, I just seem to always find myself doing a bit more scraping/sanding after installing bindings and that inevitably leads to another pass (or two, or three, or ...) over the entire surface, which has resulted in issues on a couple of tops. Spruce doesn't give me any trouble, but I find cedar and redwood to be finicky.

This thread reveals two things: The pad of my RO is probably too soft; I've been impatient with my hard block sanding. This is useful information!

Thanks again,

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George :-)

I do the Random Orbital sander
Also people how many of you control the RH if you don't you will always have them. If you sand at 45% you will be much better off than sanding at 60%

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John Hall
blues creek guitars
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You Don't know what you don't know until you know it

This has little to do with the soft/hard problem, but when I raise the grain using a thin cut of shellac, it highlights the uneven areas. I even those out, and apply another coat of shellac to raise the grain.
I used to use water to raise the grain, and it took three or four applications with light sanding in between before it quit fuzzing up. Using a thin cut of shellac (may a 1/2 lb cut), two applications is enough. I usually end up doing more just to get the surface even enough.

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Roger, that is interesting. I'll give it a try

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Joe Beaver
Maker of Sawdust