Surely at those skin thicknesses and frame/stringer pitches the whole top of the fusleage is 'No Step' without writing it over the whole thing?
Not forgeting that 'No Step' for a 10 stoner in trainers being careful to step on rivet lines is a bit different than an 18 stoner in hob-nailed boots, sticking his boot in the middle of panels... so take the worse case and placard for saddles and planks or a cherry-picker.