same here never used a clay bar before so while I was at the shop n shine event last weekend I asked them for some info on the matter.
The young chap told me there are 3 grades,smooth,medium and course. He recommended the medium one.
He said to wash the car then use the clay bar on one panel at a time keeping the clay wet with a spray of water or your usual car wash mixture. Then pollish followed by a sealer.
I bought a Bilt Hamber clay bar as was cheaper the the alternative but he said they're all pretty much the same.
Not had time to use it yet though
.
Steve