Originally Posted by Don Woodson

Is that rim made of aluminum or steel?
I'm not a bicycle mechanic, but I am a jet fighter mechanic and have lots of experience with different metals.
If that rim is made of aluminum, you don't want to rub it with steel wool. If its steel, go ahead.
If you rub steel wool on aluminum, it will look good for a few days, but what happens is what we in the aviation industry call galvanic corrosion. It's when two dissimilar metals are allowed to contact each other for any length of time. When that happens, a tiny electrical charge occurs, causing one metal to become a cathode, and the other becomes the anode, meaning one metal will cause the other to corrode rapidly. When you rub steel wool on aluminum, you leave traces of steel on the aluminum, which will eventually cause corrosion and pitting, and in this case, the scratch could eventually turn into a crack.
A more suitable option for that scratch would be beartex, or aluminum oxide sand paper. And if you're near an aircraft supply company, you can get an acid called alodine which treats the surface of newly worked aluminum and prevents corrosion.
But like I said, if it's steel, go ahead and use the steel wool.