Most probably already know this, but some minimal preparation is needed before you paint. Usually just a good cleaning with rubbing alcohol or brake cleaner of the roll marks to be painted is sufficient to remove existing oils, solvents or other gun cleaning chemicals that may be present. Paint always sticks best to a good clean non-oily surface. I use a gun patch wetted with one of the above and rub over the roll marks and then let it air dry for a minute or two. Make sure all fuzzies and lint are removed as well. Then begin to paint.
ETA: I went through this entire thread and tried to capture every relevant input from all the posters into one chart to show how to use, clean up and take off any type of material listed in this thread to date. Some small things may have been omitted, but I tried to capture the more frequently used techniques and typed it and saved it as a jpeg file for anyone to copy if they wish.