Since we're on the subject, can anyone identify what's on the right side of this block. I bought a kit used off of here in otherwise great condition. I took the block apart to be on the safe side, and I'm glad I did. Here are the before and after pics:
I tried water rinses originally, and it only got that grease looking gunk off, but the right side stuff stayed. I just soaked the copper piece by itself in 5% white vinegar (no o-ring or other parts) for about 3 hours, scrubbing at the deposits with a soft bristle tooth brush every 15-20 minutes. After the 3 hours, you get the after photo.
There is still a little tiny bit of that tan crap, as you can barely see. The second image is rotated 90 degrees clockwise. I don't know what the stuff was, but it reminded me of calcium buildup/precipitation that I used to have in my saltwater aquarium because it was very hard, hence why I soaked it in vinegar instead of trying to scrape it off. Made me wonder if the previous user used tap water in his loop, or if its just a common issue with cooling loops over time, or if there is normally galvanic transport between the brass of radiators and the copper of the waterblock (and possibly in aluminum impellers of pumps)?