You don't want it to be perfectly smooth or the paint won't hold. 1500 should be more than adequat. I actually would not sand more than 1000 before painting. Then you fill what's left w/ 1000 grit with paint. Keep more undercoat and basecoat/paint layers until you can sand the paint smooth. Then cleacoat.

I get custom mixed stuff at a local autobody supply. Just give them the make/model/year of your car along with its paint coat (found under the hood at the drivers front strut mount) and you can most likely get a custom spray can.

Between coats I sand w/ 1000. First thick clear I sand with 1000 and then 1500 thereafter. When its done I sand it all with 2000 and then use Meguiars (step 2) polish to get it to really gloss.

As for chipping it has a lot to do with how patient you are for thinner layers, a good adhersion promoting basecoat and good curing time. More thing layers are better than less thick layers.

jason