The roof on my 73 is pretty bad. Scaly and pitted. I'm using a 3M fiber wheel and a sandblaster. When I did the 66, I didn't have a blaster, so it was just the fiber wheel. The 66 wasn't nearly as bad, but there were black flecks that wouldn't come out no matter what. It's been painted for over a year, and is doing OK. Some of the pits on the 73 roof blast pretty clean, but others stay black. Not rust colored, just black. There are also those black flecks that don't even blast out/off. Am I getting it clean enough?