I used to use 80 grit..but now I use 60 grit much faster and gives a good glue joint..my radius dish is rectangular shape with a compound radius 5ft X 10 ft. I find that heavy duty 6 inch cloth backed sanding belts the best.
I go down to my local rent-a-tool place that rents rotary floor sanders for doing hardwood floors. They use 17" disks in a variety of grits. These are really tough (made for power sanding hardwood floors) and pretty inexpensive (only a couple of dollars each). Stick them on with spray adhesive and they last just about forever unless you catch an edge and put a tear in the paper.