A "gritty" diff comes form various things... mostly a work thrust bearing setup. The tiny balls and plates in the outdrive should be replaced every 3rd or 4th time you rebuild the diff, or immediately if it's a new to you gearbox. You can resurface the rings by hands sing the 400/600/800 wet/dry sandpaper on plate glass trick, but the balls should be replaced or at least cleaned thoroughly.
Diff rings also go bad. They can almost always be resurfaced, barring the worst scoring or gouging. Use the male diff outdrive and sand the rings smooth.
Diff balls should almost always be replaced with every rebuild. Use the cheap steel ones and replace them often. Ceramic and other fancty expensive balls are nice, but unless you are racing every week, you don't need to spend the money there. A jar of 100 3/32 or 1/8 diff balls would cost less than 12 ceramics.
Clean EVERYTHING with motor spray and dry it off well. I use a detail brush to make sure the parts are all free of dust and grit from the cleanings. I also inspect everything with a loop before it is reassembled. You can't see most of the stuff that will make a diff a mess.
I posted a lot of my diff rebuild techniques in a thread here a few years ago (SEARCH), and a similar tutorial on my
Harper Sprint Car Blog here.
The best thing you can also do before you reassemble everything is to put it all down on a clean paper towel, then WASH YOUR HANDS. Dirty hands can ruin a perfectly good clean diff rebuild, and it's always helpful to start from a clean slate.
HTH,
doug