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Use some coil cleaner. It is made to clean AC coils but works great for radiators. It is non acidic and when it sits on the coils it turns to foam expands and forces the dirt out.

Here are a few links to some

<a href="http://www.drillspot.com/products/36885" target="_blank">http://www.drillspot.com/products/36885 ... leaner?s=1</a>

<a href="http://www.drillspot.com/products/37493" target="_blank">http://www.drillspot.com/products/37493 ... leaner?s=1</a>

 

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I just started using it and so far I have had no problems. I do about a 50/50 mix of it and water in a spray bottle and just spray the radiator. Let it sit for about 5 min hose out and looks like new.