Frei wrote:
Its not a tight fit from what I have seen. Jeeez louise, I dunno...
Frei, I am not sure I understand what you're saying…
In any case there are simple jigs you can make to perfectly shape the underside of your braces to the desired radius. Either for a table saw, router table or sander. Peruse various builders' sites for ideas, it's pretty straightforward.
Sanding your braces in the dish (if that's what you mean) is not the best way IME as the underside won't be flat, and you have to make sure you don't rock the brace either while sanding. However it can be done. Just scribe the underside of the brace with a pencil, when all the pencil marks are gone, you're done.
Grit doesn't make any difference. Personally I use 40 or 50 grit in my dish, but that's to shape the rim, not the braces. Spruce compresses quite easily anyway, so it's not hard to get an airtight fit with minimal pressure. No need for feeler gauges…
Regarding your top for the "bracing" class… If it's too floppy and/or sound dead, you will learn nothing trying to brace it. Use a good sounding, stiff top. Again plenty around, sitka or Carpathian, or whatnot, appearance doesn't matter. Use that floppy sitka to make grafts…
But again, IMHO, the best way to go is to build another guitar. Bracing a top as an exercise/class/whatever is kind of pointless unless you build instruments with them…