Oh boy, I can just see everybody jumping in with their opinions on what grit to use. Again, some will tell you that you can get away with scuffing it down with some Scotchbrite, but I think that is a shortcut method that leads to poor longevity.
When I did mine, I went over it first with some 100 grit, then 220, then 300. Shot 3 light coats over that and its been holding up great every since.
And yes, if you mess it up...you can always resand it down and try again. I actually took my rails off the bike, prepped and painted it, took a couple hours break, then came back and re-installed all in the same afternoon. Get your little brother to come over and help sand and have a couple beers while you're doing it!