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I received my radius dishes and had a question about how to handle them. Since they are made of a low density fiberboard is it a good idea to seal them with a polyurethane or something before putting the sandpaper on. Also, should I expect any problems getting the sandpaper on evenly since the dish is concave but the sandpaper disk is not?

Yes. I use floor polyurathane thinned about 1/2 for the 1st coat then 2 full strength coats.

I would add that you should use solvent based coating for sealing dishes - water based products always swells composite materials and for that matter to some degree will distort any natural wood as well.

I agree with ken. I like the Mineral based poly . You can tell if is tell you to clean up with water, that isn't what you need. You want one that takes Mineral spirits to clean up with.

And the sand paper?

Unless you are going to power/motorize the dish there's really no need for a single one piece circle of abrasive. Nor is it necessarry to cover every square inch of the dish. Self stick 5 or 6 in sanding disks can be placed to cover much of the surface. Better yet strips of any width rolled adhesive back paper can be placed side by side then trimmed on the perimeter edge.