Mike,
I do not have a lot of experience with Rustoleum,, I do know from reading about people who use it,, it takes a while to "set",, so you may have to wait awhile to be able to effectivly sand it,, and most people warm the can and shake the living crap out of it ,, like for a LONG time,, this actually sounds like it was probably not thouroughly shaken to disperse the pigments before spraying,, I dont know that its dirt,,
that said,, same applies,, wet sand with 500 or 600 grit to eliminate the texture,, water with a few drops of dishwashing detergent,,