Here is my input..
First off, what you have is not a sanding sponge. It is called a gummy stone. It is very effective at removing small burrs/imperfections on the edge itself. It doesn't take off nearly as much material as a file, but in this case that is a good thing.
With the gummy, do 10-15 passes over the entire ski. This will help to minimize the almost guaranteed imperfections that skis have when they come from the factory. Next, start working the furthest contact point on the front/rear of the ski. This will help to minimize the hooky tendencies of some skis.
At this point, you should be set. Hitting rails will detune underfoot pretty fast which means you really shouldn't pay any extra attention to the underfoot section of the plank.
Mr.Bishop. -We could at least try to be mean to ski racers or something. Just lashing out against our own family seems silly and divided.