marosell if you put some neatsfoot oil on a rag or a paper towel and apply it sparingly to your strop it will increase the draw. A little goes a long way. Some guys have good luck just rubbing the strop daily with the palm of the hand. Natural oil from the skin goes in a little at a time.

The sandpaper or pumice stone will take the smooth finish off of the leather and make it a different surface entirely I think. Not necessarily a bad thing but I would try something less drastic first. BTW, the pumic stone I got at the beauty supply is aggressive. Way more than sandpaper in finer grits. As sffone said, I think the paper works better too if you go that route.