when brakes get darkened up (rotors), what kinda polishing can you do and how long does it last? can you get pretty close to the original shine?

I don't have BBk at the moment, but might considered a used set sometime down the line..

 

You could use some steel wool to remove the corrosion, but it will return. I have a new set of inexpensive rear rotors that I will painting soon. I will be sure to post photos.

 

Use brake cleaner (available at most auto supply stores) to remove brake dust/residue. Do this when the brake rotors are cold, (not driven yet). Jack up the car and remove the tire/rim. Spray the brake cleaner on a lint free towel/cloth away from painted surfaces (brake cleaner can damage painted areas) order to remove any brake dust/residue first.

If rotors are still dark, it may be due to wear/heat. Should be ok unless if rotors are warped.

Next, re-install the tire/rim and torque the lug nuts to spec (I seam to recall - its 89ft/lbs) for stock rims.

Now if its the calipers you are referring about being dark; then clean the calipers when they are cold (not driven yet). I use a citrus cleaner/warm water that is compatible with the EB coated calipers that is environmental friendly. Note: if calipers are painted; brake cleaner may ruin the paint!. If calipers are stock (plated I believe), then most mild cleaners should work. Try cleaning with a old toothbrush in order to get into the little calipers areas and rinse with clean water. If you barake calipers are painted, toch up any paint chips and wax only the painted caliper block surfaces, not the rotors.

After cleaning, drive around the block to wear off any cleaning residue off the brake pads and rotors.