I have the multiple shape sanding tools. I want to use the 1/4" radius feature. I also have the sticky backed sandpaper, but it doesn't work very well with the tool. Any other suggestions? Is there a way to make this work I don't know about? TIA.

Carl Goldberg used to include one with their Ultimate biplane. I think I still have one I see if I can find and post a picture. It was just two pieces of flat ply and a U shaped bracket for each end you glued the flat pieces to. Then stuck the paper on. They work pretty good. Overall it ends up about 4" long and 1-1/4" square. Every kit had one, maybe someone close to you has one.

David

I have a number of "PermaGrit" sanding tools. I think they are still made? My collection includes flat, round, V shaped, concave, and curved.

You can easily make a selection of concave sanding tools out of short lengths of PVC (or other material) pipe.
I've made a number of them using pipe of different diameters (PVC works best and is easy to cut).
Simply cut to desired length...you might want some long (like for leading edges), or short for utility work.
Next cut in half, quarters, or even thirds lengthwise..
Use sticky-back sandpaper of the grit appropriate for the task.
Bonus: If you wanted to sand a concave surface on the plane then just put the sandpaper on the outside of the piece of pipe.
Cheers,
G42

I use 3M 77 Adhesive spray and regular (non-sitcky backed) sandpaper. Will keep your sandpaper firmly attached. When changing sandpaper, peel off the old, wipe the tool down with mineral spirits to remove the old adhesive, and you're good to go.

Here is a sample of a convex sander. Cut in 2 lengthwise and you can make a concave sander. I also use the plastic cover from caulk tubes. Smaller diameter is doable. You just have to find the right size tube.