As featured on OC
here
I'm bookmarking this, so if you're a noob, bookmark it too, 'cause now everyone knows!
In short, to clean a copper block, you can use ketchup, vinegar, salt, flour, toothpaste, ...
Personally, I don't think this is very thought out: We don't want acids to clean the inside of a waterblock, because it may cause pits in the copper, which would trap water, and lower the efficiency. However...
If we load up a rig with water, keep the pump running, THEN add the acid, it might change the pitting into scarring, which would be favorable to cooling (at a very small scale, but it's there).
The question then would be: how do you dump the acid without stopping the pump? We could neutralize the acid with baking soda. (now if you were a tinkerer as a kid, like me, you know what happens when vinegar is mixed with baking soda: it foams up!).
This is going to require more thought.