TireRack (www.tirerack.com) rents out a fender-rolling tool. It connects to your wheel studs and has an articulating arm with what looks similar to a urethane skateboard wheel on the end of it. I know some Subaru guys that have used it and it works well. I would suggest heating up the paint with a heat gun to avoid cracking. The low tech way to do it is to wrap a baseball bat in electrical tape (to prevent damaging the paint) and use that. Both ways work, the tool is probably the better/safer choice, though.
Good luck.
- Trent
