Very rarely have I ever used tips of the fingers to sand. Also I don't ever think I had the tips sensitive from sanding either. When I would sand along a body line I would use the area right at the base of the thumb to sand with. I'm not sure what you would call it, maybe the ball of your hand. If I had to get in a tight place like along a drip rail I would crease the paper, insert it into the area and use the thumb to sand keeping pressure light. Everyone has a technique that they use and it is kind of hard to explain how to do it. Also when you sand make sure you keep the paper wet to keep it clean. Usually I have a bucket and a sponge to keep things wiped down and keep the paper dipped between sanding to keep it rinsed out. That makes the paper go farther and makes the sanding easier. Of course quite a few people will sand dry. That is your option. The only thing you really need to be sure of if you wet sand is that the car is thoroughly dry before painting. Water can stay trapped in cracks and crevices where you would not expect it. I always would take the air hose and blow out all areas just to be sure. Nothing can ruin a paint job faster than water coming out of a crack.