Isn't this painful for the rodents as you "etch" the skin?
i've tried by staining with picric acid. its supposed to be in powder form. But the one I have is in solution. What I did was I dip cotton bud into the picric acid, and stain the mice according to my number coding (right front hand = 1, left front hand = 2, right flank = 3, left flank = 4, head =5). I tried this last 2 weeks, and the dye still there and all the mice are ok (no sign of abnormality). just make sure they don't lick the dye before the dye dried. you can apply it by holding the mice by tail.
hope it helps! =)
and powdery picric acid (i.e. dry) is an explosive, so you should be extremely carefully...