ONLY use the 600 if the original scratches are to deep to remove with the 1000. You said that it looks like someone went over the hull with a Scotch Brite pad. These pads are a hell of alot more abrasive than 1000 and even 600 for that matter. For example, you cannot remove 80 grit scratches with 320 grit. Not abrasive enough. You would have to go to 120. Then 240 to remove the 120, 320 to remove the 240 and so on. You get the idea. So, depending on how deep the original scratch are depends on what grit you will start with. As stated before, start with the compund first and see how that turns out. Then adjust from that point.
600 does seem kind of course, but the sides of my hull were in rough shape. They actually felt like sand paper when running your hand over them. Now they are like a mirror and smooth. Every piece of literature I read on gel coat repair had a finsh grit prior to buffing of 600. That is how I made my determination on what grit to use.
I WOULD NOT wet sand the corners as Meguires eluded to. Too risky.
The buffer you are going to rent sounds like the wrong type if it is an orbital. It needs to be a rotary. The orbital has a sort of dual motion (like a sanding DA) of spinning and vibrating.It does not build enough rpm to be effective with the rubbing compound. The rotary buffer only spins which will have enough rpm.
I am not an expert, but have experience in this. Some good and some bad, but that is how I learn.
If you want give me a call at the office or on my cell.
cell 773/837-1008 After 10a.m.C.S.T