IBFlames

So last year I had lots of clear coat fading on my cars roof. I sanded it down and painted it myself using single stage paint. It turned out excellent, but this past year I've been at college where the car was outside letting the elements do torture on it. So now there are permament 'water spots' on it. I've washed my car about 3 times in the past 2 weeks and waxed it a couple times. The paint is starting to disinergrate(Yeah, I know I spelt that wrong). Today my dad told me that I should go over to Maacos and get a quote. I was going to get my whole car painted if it wasn't a lot of money but decided against it. I talked to the owners for a while and when I left, I had an appointment to get my roof painted by them on Tuesday. I just need to do the sanding and stuff on it before hand.
I'm planning on using 600 or 800 grit wetsanding it down to the primer. What grit would be best?

Oh and its going to cost me $119.
Please don't flame. I know its Maacos, but my sister and a few others I know have gotten their cars painted there. My sister got hers done about 3 or 4 years ago and it still looks new. The owners are certified by Maacos, which very few are, only the very best, and have been in the auto paint business for years before hand. Very professional work done.