A couple pieces of advice from someone who has rolled dozens of fenders...
First clean the surface the roller will contact and make sure the top of the fender lip is clean as well. On top of the fender lip there is usually sand, gravel or chunks of asphalt sitting up there and if you roll the fender hard enough of its self you can actually create small waves in the panel.
Secondly, use heat and take your time. Check the weather and wait for a warm day if possible and use a heat gun, your girl's hair dryer can work in a pinch. The heat will help prevent the paint from cracking, but the older the paint the more likely it is to crack...
You can also go a step further after you've rolled the fender lip flat and actually pull/flare the fender out for additional clearance using a proper fender roller like an Eastwood. Always take your time and make small adjustments to the roller...