I did an old Duster fender last night using this method and also using a spray bomb of rustoleum. I just wet sanded them and applied the next coat and can tell that it does have the potential to do a decent job. To be honest both the roller applied and sprayed on parts of the panel look decent. I did not do a lot of surface prep I really just wanted to test the concept out. One key seems to be the roller itself. The high density foam one that is white with the rounded edges. I was having trouble getting it applied to the contours so I tried doing the spray and then immediatly applying around it with the roller and I cant tell where one ends and the other begins.

On a side note a few years ago I painted a Harley tank and fenders with Rustoleum spray and then used the clear coat and the thing looked awesome.