I always find that the insides of the arms never tan very well in comparison to the rest of the body, but then people have less melanin there anyway, so this part of the arm wouldn't tan in the sun!
But my routine for arms is to lightly spray the fronts of them while I'm spraying the front of the body, and then the same again when they're facing the back..
When they turn back round (So just before I spray the face), I get them to do a series of arm movements;
Hold their hands out, palm down with fingers spread, so I can lightly mist the hands, and go over the fronts of the arms again
Hold their hands out, palm upwards, so that I can spray the inner arm (I always mask of their palm with my gloved hand, just incase!)
Turn their hands so that their thumb is ontop and lightly spray the thumb and upwards up the arm
Turn their hand over, so that their palm is facing the outer wall of the booth and the back of their hand is facing their body, so I can lightly spray the little finger and up this part of the arm
Then into "muscle man" pose, as if they were flexing their biceps! (So holding both arms up at right angles to the body) so I can check the inner part of the upper arm,and underarms
Then finally to drop both hands as far down behind their head as they can, so their elbows are uppermost- this lets me check the back of the upper arms, and also lifts the breasts so I can spray under there of need be...
Very long winded answer!!! It's much quicker to do than to write!!
Hope that helps a little
XXX