I use a magnetic tankbag all the time. Mines a Joe Rocket. I have a towel between the tankbag and tank and have no scratches on my tank from it. I also polish my tank all the time. The bag stays in place. However, if the towel gets dirty or the magnetic flaps get dirt under them, the tank will get scratched. Keep the towel clean (if you even want to use one) and the flaps clean and you should have no problems.