To get to the rear of the trigger bar, I have a modified dremel attachment. I cut small narrow strips of sandpaper, then hold it on the dremel attachment with a very small o-ring. Makes changing out the sandpaper strips a snap. I'll post a pic of this little tool.
On the main trigger bar and exposed areas, I use 400/800/1000 and keep it wet with WD-40 (I buy WD-40 in the gallon can). Also remember it's critical to alternate your sanding pattern, up-down, then side-to-side, but go lightly, not much pressure. This alternating pattern will produce the desired level of polish quickly and efficiently.
Lastly, I have a super fine polishing compound I use a small felt dremel wheel with for obtaining a glass like finish. Polished metal is slow to rust since the pores of rough steel are essentially eliminated. Just a light coat of lube will keep the polished parts rust free.