The best thing I have found with coverstocks like this (a ball that should clearly be used as an oil ball for most) that works best is to adjust the cover before you even drill it.
For instance, what I did with my Fear Factor was I wet sanded it with a green scotch brite pad (about 600 grit). What this does is gives me the exact reaction every time I use it. Once I see that the cover is taking in a little oil, I lightly sand it with the pad that I used on it in the first place. Some of the stuff that comes dull I use a burgandy, some I use green others I use the grey scotchbrite pads.