just remember that ANY paint job is only as good as its prep work. use a good quality filler to smooth out the fender first. then put a decent coat of primer on it, and make sure that's smooth, hit it with a light coat of primer in a contrasting color (a guide coat) and sand it till most of that color is gone, any of that color thats left will reveal low spots. Fill them with primer or filler depending on how deep they are. It will take you forever if you are trying to fill low spots with paint. If you keep sanding down to bare metal, you probably have some high spots that need to be knocked down.
Now, with that out of the way, this question goes out to the guys using the foam brushes and such, do you have a compressor, or know anyone that does? I can see using rustoleum paint to save money, but I can't see the appeal of brushing it on and having to do that much sanding, and taking that much time. Unless you get lucky enough to get a super glassy finish the first coat like Bodain did (AMAZING job by they way) and that's not a likely result by any means, you are SO much better off doing like Phat and Shizzo did, cheap harbor freight spray gun, if you are going to be thinning the paint down anyway, spray it and be done with it. If you don't have access to a compressor, use the spray cans, if you do it right, you can get a great shine without ANY sanding, and an AMAZING shine if you do sand it, and you'd only have to sand it once or twice.
The brush one, roll on deal OBVIOUSLY works, but its creating WAY too much work, and taking WAY too much time