I like Bar Keepers Friend (Sold in Supermarkets for about $2) It's chemical free. Plus its been around since 1882
I hope your just cleaning the top and sides, because if you use any polish on the heatsinks base, the polish will kill your lapping hard work. It would be foolish to fill the microscopic pits and valleys with polish.
They claim you can get a free sample, but I cant see how, perhaps you need to e-mail them.
It's a common ( and potentially fatal in this case) error to confuse shiny with smooth and flat.
It would be a piece of cake to hit your HS with a buffing wheel and make it shiny, but if it's not flat and (relatively) scratch free then you've accomplished nothing.
If you were referring to the finned areas of the HS then that's a different story.
Copper stains/oxidizes very easily and the shape of the HS makes it very difficult to polish out, although it could probably be done.
A good cleaning in a ultasonic bath with the proper solution would be about the most painless method I can think of.
Most modern clock repair shops have exactly the setup required to do this, perhaps you could see if a local shop would be willing to clean yours for you.
That would get you as clean as possible.
Depending on the design of the HS, tumbling it would also achieve a very nice high polish, but I know that my Zalman wouldn't survive the process...the fins are way too thin and fragile.
No polish is "good" to use on a finely lapped heatsink. It will most likely fill up the small valleys in the heatsink with oil if it is an oil or grease-based polishing compound, and if it isn't then you will most likely leave a fine coating of polishing granules in the cracks, preventing the thermal compound from doing it's job.
If the base is REALLY oxidised then consider lapping it again with a fine grit paper, if it has just gone slightly more coppery coloured then don't bother. The oxide layer is probably going to be less that 1 micron thick if it's just discoloured.
Im refering to the base of the HS. I have wet sanded the HS wit 600 grit sandpaper and washed it clean under the tap. After I noticed the base is discolored. Also i have read several lapping articles using polish to make the HS smoother and get a mirrored shine. Now I'm confused. Should I dry sand so it doesnt get discolored?
The discoloration that you see is irrelevant.
Your chip has no vanity and doesn't spend any time looking at it's reflection, so whether the base is a "mirror" shine or not won't matter one whit.
The slight staining that you see on a freshly sanded and washed copper surface are just chemicals in the water and shouldn't impact the HS's ability to conduct heat in the slightest.