Don't use Dr Downpipe until you have seen this 

Totally agree with you about the Harpic , it will 100% do the job but its the after affects that worry me ,Dr downpipe looked decent stuff , I`m still using muslin cloth and metal polish if it gets too bad then a scotch-brite pad , not an easy job but once its done if you keep on top of it its easy to maintain , just a bit of a pig on the tiger sport to reach the middle header behind the radiator , great work mate