Car Toys
+1y
The proper way would be to roll them prior to painting them, lol. J/K, they used to make a tool that bolted to the hub assy on the vehicle and had an adjustable arm w/a large nylon wheel on the end of it that would roll the fenders. Not sure if it did it w/o damaging paint though. The stress of the metal being reshaped and the limited flexibility of the paint would probably play a big part on whether or not the paint cracked or flaked while you were rolling it. I honestly cannot remember who made this tool, but you might be able to search for it online. Hopefully you will come up w/something. Sorry I couldn't help more.
Late