[Titan151]

Just electrically isolate the radiator or waterblock from each other. This will eliminate the need for painting the inside. Electrical isolation of the two metals will eliminate the metallic path (one part needed for a reaction to occur between the two). Without this path, there won't be any issues between the Al rad and Cu block. In most cases I am willing to bet that the waterblock is electrically isolated from the case. With a mobo removed from the case, I find no electrical path to any of the mounting points. This however may not always be the case depending on how you secure the WB to the mobo. To be sure the block is isolated check the resistivity between the water block and the case. If there is no electrical connection, their is no metallic path and the potential for a problem between the two metals is eliminated.

Also, your idea for internally coating the rad is not a bad one. This is done in all sorts of places. They either pump or blow a mixture through piping. I have heard of this being done with concrete to internally line irrigation piping. But to do that for a rad might be overkill, and as someone mentioned reduce heat transfer.