I didn't use a scotch brite pad myself. The scotch brite was removing the roughness of the fenders. I just used a rag, with soap and water and scrubbed them really well. I checked them for any black build up, like I had on my black instrument knobs with my fingernail. I did have some especially in the front folds. So, I cleaned it well, and I may have used a little acetone to make sure that no embedded dirt was on them. I dried them and I used the Rustoleum Satin Black "Paint and Primer in One". It took me two cans. Make sure to tape well, and paper the entire hood and fenders. That black paint will float and fall everywhere. It is best to remove the fenders to paint them. After two years, mine look great with no peeling.
That discoloration that you see is likely the result of armor all. It seems to cause bleaching of the plastic over time. I only use it on my tires now for that reason. I definitely don't use it now that I have painted my fenders. It immediately gives them a bleached looked. I have been using a little WD-40 on my black and it seems to work better. Just not too much.
Here's a picture of my painted fenders after two years. The white on top is just the light.