Easiest way to smooth the surface is with a "Scotchbrite" or other nylon scrubbing pad and "Softscrub" or other fine abrasive cleaner. This will remove the black stuff and make the surface smoother at the same time.
Wash with hot watter and dishwashing detergent to remove the stronger cleaner, dry well.
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I actually contiued to work away with the alcohol and lense cloths last night and got it looking decent. I put the Arctic Silver on and let it run with the BIOS heat monitor screen up. The CPU temp stbalized at 50C, my case temp was 35C. I know that that temp is really high for a 2.4C, but my case temp seems high too. Is a 15 degree differential a lot or is the heat sink doing a decent job based on the conditions?
Regardless, I'm planning on taking the heat sink off again and doing some of the things you recommended, we'll see how it works.
Reasons not to use petrol.
A. it stinks
B. Its carcinogenic
C. It contains all sorts of nasty compounds
D. It contains heavier compounds that dont evaporate right away, leaving crud behind
E. The remaining crud could hinder cooling, or vaporise when the cpu is turned on, giving off bad smells.
Go for clean solvents. Metho, lighter fluid, acetone. (though personally i dont recommend acetone)