An 8" softpad with 80 grit is a fine and correct tool to use, but not with a grinder.
"Grinder" is 5000 rpm or more
"Sander " is 2500 - 4000 , more or less
"polisher" is 0 - 1500 rpm.
A polisher is what one should be using on thin red cedar.
Also, all softbacks are not the same. they vary greatly. This tool is a polisher, It can run VERY slowly. And here are 3 very different softpads. One has been cut down to about 5 inches. Gluing 6" paper to a 5inch softpad allows one to sand concave areas very well. And before we get old.If disc sanding was better understood, we would not have to muck around with the concave /convex router business.