I cut my painted fenders and didn't use the black edging. I didn't have any problems with the paint wanting to come off. It takes a little more time to smooth out the edges for a nice finish but the result is very nice. Here are a couple pics before I put the finishing touches on.
I taped the line I wanted to cut with blue masking tape before cutting, just eyeballed it. I used a dremel tool and cut on top edge of my tape (that was my line). Follow the video on project-jk about how to chop the inner front liner.
Here are the tools I used.
After cutting with the dremel there is a lot of plastic and it's rough. Used the below wood rasp to smooth things out and get a nice straight edge.
Used this vibratory sander with medium grit to get things all nice and smooth. Then moved to a fine grit to really finish things off. I'll get a pic of how they look now.