I usually start with 800, but accidentally got a quarter sheet of 600 mixed up with 800 and was surprised to see the big scratches in the old paint in the blend area. But this showed me that the new paint that had been sanded with 600 showed no scratch marks after buffing. I have seen two ways to buff out sanded paint, the "dry" method picks up a bit of compound and buffs out a small area. I was shown the "wet" method by an old car painter where I squeeze out a ribbon of compound about 3 feet long, then dip 2 square inch rag in water, squeeze it a bit and then spread out the ribbon to make the area about a foot deep X 3 feet long. Starting with a 4" bite, I make repeated passes left to right starting with increased pressure, the backing off the pressure until I not have only the weight of the buffer pressing down as the luster shows through. More cutting in obtained by increasing pressure and/or, by using more water. I use 3M rubbing compound. E-mail me at johnsinclair210(at) yahoo.com and I'll be glad to send you photos of a couple ships finished by sanding out Concept Acrylic Urethane while the paint was quite new.
JJ