On wet sanding paint work, there's definitely a bit of an art to it. A lot of it depends on how rough the surface is to start with. 320 and 400 will remove big imperfections quickly but do leave scratches, which require more wet sanding to remove. So I pretty much save that for between coats sanding, unless I am going base/clear in which case I will use 600 between coats. The final coat before the clear gets 1200 grit just to even it out. I then wash the snot out of it, dry and tack rag thoroughly and shoot my clear quick so it doesn't have time to pick up lint etc.
I always sand with the long axis of whatever panel I'm working on. That way if there is a scratch the eye doesn't pick it up so easily.
The problem with this whole wet sand system is that no matter how fine the compound etc is, it's just about impossible to get that hard clear shine of a non-sanded clearcoat, it will always have a sort of a haze to it unless you wax it and this is particularly bad with dark colors. I am lazy and don't like to wax.
OTOH, single stage generally looks real nice after wet sand/buff even without wax.
I have very little experience with wetsanding/clearing aluminum in any form (I have only done two side panels/number plates and was purposely going for a brushed aluminum look) so I think 5twins will be much better equipped to answer that one.
FWIW, I did that sort of backwards: I cleaned the panels thoroughly, used 600 grit to make sure it was smooth, then used 320 in one direction only (no back and forth) to give it the 'brushed' appearance, then cleaned with acteone/tack ragged again and shot clear quick, before the aluminum could start to oxidize. And it will oxidize QUICK, or at least the unknown alloy I used did.
