I looked up what 600 grit would look like and got this.
http://www.arscives.com/bladesign/Images/b.600gritw.jpg
To get something like that would easily take another five hours of work. I never thought of my skin as being weak, but I already have a blister from sanding down to 150 grit. Is there some sort of machine you use for this?
Also, when you say 600 grit for primer, do you mean that 600 grit is smooth enough for paint, or that 600 grit is necessary to begin adding primer?