My son drives a Holden one tonner V8 ute...no not a VW i realise but it is watercooled same as many of you and therefore has a radiator. The ute lately has been overheating and when i looked in the radiator i could see mineral buildup(and a lack of green fluid) in the cores. I drained the system and blocked the lower hose and filled the radiator with CLR diluted 3 or 4 to 1...lefte overnight and drained it and repeated a few nights. Each time i drained it there was a lot of scale and little pebbles of mineral which were nearly dissolved. Put it back together and bingo cool as a cucumber! This was a full copper radiator and i was prepared to sacrifice it...but no need. simple fix for about $14!