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I'm working on a project that has a piece of curved (concave) crotch walnut. The curve is in three dimensions and is roughly 8"x6" in x-y and curves inwards roughly .5" in the z axis. I used a spokeshave to get down to the final shape, but due to the grain direction of the crotch walnut it was very difficult for me to get a perfect surface finish.

That leads me to sanding. What's a good way to sand a surface like this? Obviously I can just back the sandpaper with my fingers, but I was taught that this is poor technique. I need grits down to 120 or so (a few light tool marks left that I can't get rid of with the spokeshave). I was thinking that an interface pad to my orbital sander might work, but I'm concerned that the paper will fold and effectively sand some places, but not others.

Google shows tons of results for flexible sanding sponges, but I can't make heads from tails from all the different options. Several appear to be made of different materials than the sand paper I'm used to and are rated "coarse" or "fine", but don't state grit. I'm sure there is some subjectivity here, but I'm curious what the pros/cons of the different options are.

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I'm working on a project that has a piece of curved (concave) crotch walnut...due to the grain direction of the crotch walnut it was very difficult for me to get a perfect surface finish.

Instead of sanding, I would suggest using a curved scraper. When properly sharpened, they work very well on weird grain directions and leave an almost polished surface behind.

Lee Valley (among other places) sells scrapers like this that would fit your bill. You can also make them out of old hand saw blades and file them to the profile you need.

scraper

Obviously I can just back the sandpaper with my fingers, but I was taught that this is poor technique.

Not necessarily. If you're trying to get a true, flat surface then yes, backing sandpaper with your fingers is not a good idea. However, for your case it will work fine since you're trying to get into a spot that "normal" sanding techniques can't get into. If you want to avoid the subtle dishes that can occure from using your fingers (or keep your fingers from getting too hot), you can put a piece of leather between your fingers and the sandpaper to spread the force out more evenly.

I was thinking that an interface pad to my orbital sander might work, but I'm concerned that the paper will fold and effectively sand some places, but not others.

You might consider buying or making a sanding ball. These can be quite effective.

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  • OP's question is I'm working on a project that has a piece of curved (concave) crotch walnut. – grfrazee Oct 2 '15 at 14:07
  • I just feel shame now. :( – Matt Oct 2 '15 at 14:08
  • Hah, no worries. I always read these things too fast the first time around, too. – grfrazee Oct 2 '15 at 14:09
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Obviously I can just back the sandpaper with my fingers, but I was taught that this is poor technique.

It is poor technique, until it's not. This is an ideal situation for when sanding with your palm/fingers is a good solution — one of the great things about the hand is that while not exactly infinitely adjustable it is multi-conformable, capable of sanding concavities as well as convexities.

I would however urge you to try scraping as the first port of call; and not just here, as a general principle. A well-sharpened scraper* is nearly unbelievably faster than abrasive paper, doing the job that rounds of sanding would do — 80, 100, 150 — but in a single operation. And in a fraction of the time. Perhaps best of all, little or no dust!

You're almost certainly going to want to do some sanding afterwards (despite their reputation for producing a finish-ready surface it's very common, if not typical, to lightly sand after using a scraper) but it should require a minimal amount using only the final grit you'd use prior to applying the chosen finish.

*Note that scraping can be done with two types of edge and work effectively. The first is the conventional burr, the second is the knife edge. The burr is usually turned with a burnisher but see this previous Answer for a quick thing about heretical sharpening, as well as other info on scraper use.

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If your curves are not perfect circles, which they don't seem to be from your use of the word wide, then I would suggest a sanding star. These would conform to the existing shape better if that is something important. These might be too big for your surface but perhaps a dremel attachment exists.

Sanding Star

On the off chance your curves are more uniform you could get away with a flap wheel that was the same diameter.

Flap Wheels
(source: mig-welding.co.uk)

  • I would try to scrapers first though. – Matt Oct 2 '15 at 14:24
  • Having recently used a cabinet scraper for the first time: whether it's the best tool for this task or not, I definitely recommend making an excuse to try them. – keshlam Oct 2 '15 at 22:53

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