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There was a recent thread about radiusing fingerboards. DZSmith mentioned using an air sander (inline) to do it. Here's a pic of what I put together. I've had this sander since the '70s (made waterbed frames then). Velcro, 16" radius block, manzanita fretboard ... 10 minutes.

Is it level? I'm not sure I would trust a short radius block to that task.
Just sayin.

Cal

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Remember, there are no stupid questions, only stupid answers!

I've used a short block to get the general shape and then drawn pencil lines across the fretboard, and using a two foot level with sandpaper glued to the face of it, sanded along the string paths to make sure there weren't any dips in the board.

What did you use to stick the wood block to the sander?

Before I started sanding I had a white line down the center and lots of crosshatches so felt pretty comfortable that it was sanding evenly but I will get a set of longer blocks. I think if you swapped out blocks while doing this a compound radius could be done.
Clay, I used a piece of 2 piece Velcro that was the size of the block so swapping out blocks would be easy.

Drawing lines and crosshatches will not ensure a straight fretboard. Like Clay, I also use a 24" long sanding caul.

But, I use a similar setup to your sanding machine to smooth and straighten the back of a neck. My tool is an electric pad sander with a 10" flat sanding plane screwed to the bottom. It does a quick and good job of it.

I had a request to post of photo of my back-of-the-neck sander. The sanding beam is 10.5" long and I use 100 grit paper on it. It is attached to the sander with 4 screws. The aluminum sanding beam came from a hobby store for sanding model airplane wing spars.

I use the sander in a shoe polish motion on the back of a neck to smooth out the shape after using a rasp. It leaves a surprisingly smooth surface that only needs light sanding. It also evens the neck shape nicely.

Carl
I like it. With the cautions you are taking to keep it from cupping, I bet it is coming out pretty darn good. As you know you can always double check with a straight edge.

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Joe Beaver
Maker of Sawdust

Yesterday I had reason (verifying neck angle on a build project) to revisit my fingerboard shaping and I clamped the fingerboard to my granite surface plate that I finally assembled and put into the wood shop. The things you find when you think you're done with a part and go for 'one more look'. The surface plate wasn't set up when I shaped that fingerboard.

I think getting the fretboard onto a dead-flat surface before sanding/shaping will pay me dividends when I get everything all glued up and fretted.

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Peter Havriluk