I did it about 6 years ago.. Used kitty hair as mentioned with a bondo skim coat. But instead of doing the sides I just took the panels off and shaved the hooks for the panels and holes.
The best way to do it is to fill the channels with evercoat polyflex and sand that down.
The problem with the side panels is you can't really go about sanding them on the car becasue they're flimy and will deform. And if you look at them closely on the car you can see that the clips etc holding them on are actually bending the panel every few inches. So, even if you were to take them off and get them flat off the car, it'll be wavey when you put them back on due to design. And if you do take the side panels off have a stockpile of spare clips ready because most of them will break.