Zinc plate it... i've done a bunch of axles and all came out beautiful... if not that, then polish it w/ some sandpaper
Got this from GRK...
Find a drill bit that fits tight in hole at end of axle.... run the drill w/ one hand (in the carrier, in the swinger) and hold some 220 sandpaper on it...then go 400, then 600, etc as high as you want.... sort of like a lathe w/ the drill
That will clean it up pretty good, but it'll still get surface rust w/ time... just have to keep up w/ it