I have always taken a clean cloth, soaked with enamel thinner, and wiped down the area I was going to prime or paint. I was just told by the body shop supply that you can't do that on bare body filler as it will soak in and ruin the primer. I did some research and found a large article by a guy who works for "rage" the company that made my filler. He says you must warm the area prior to priming to drive out moisture from the filler but nothing about cleaning prior to prime. Otherwise you may get a ring around the filled area. Yes, I've had those rings and did not know what caused them and am surprised the filler absorbs moisture from the air. I saw so many people cleaning with thinner over the years I wonder if it is OK now? If not thinner, how do you degrease and clean these days?