check how the grit is affecting the scratch as well. It will tell you a lot about the adhesion. I also like to take door, hood, decklid edges down to at least primer or bare metal. If you block it out really nice with a 240 or maybe even a 220, you can go back over the whole car with a DA and an interface pad with 320 then again with 400-600.
The main thing is finding what needs to be fixed. I usually clean the car really good, get a good light on it, then sight down the sides of it. Then I fix what I can see by blocking on it and filling. If it's a new car with one paint job(assuming) I'll use a da to feather the area and reveal lows and highs, which works great cause you know exactly where to apply filler for tiny stuff, but the DA prep isn't always the best when you have layers and layers of stuff, unless you want to break it open and bring it to metal. Anyhow, Once I got those areas I can visibly see fixed and filled I guide coat the whole panel and see what that tells me. Depending on the color and gloss on the car some more stuff might reveal itself. I'm currently working on a yellow dull superbee, and if you were to look at it before we worked on it you'd never guess filler needed to be added in the areas we applied it to. The color and gloss on the car made it incredibly difficult to find what to repair. I used guide coat and the man next to me didn't. I had to go over everything he did when he wasn't looking...our secret.