1000 sounds like a safe bet. I can't imagine any paint so thin that scratches from that would show through.

As far as sanding improving adhesion, I'm not sure about that. I always thought that wet sanding between coats was more to smooth out slight imperfections in the previous coat than for purposes of improving adhesion. After all, when paint is sprayed on in many layers, nobody ever wet sands between coats, as far as I'm aware.

But like I said, I'm no expert on that and will wait for someone else to chime in.