How do you clean exterior fins of car's radiator?

First of all use an airline blowing from the rear through to the front of the radiator,then using a brush ie paintbrush ,on the front fins to gently clear any loosend debris but not forcing them further into the radiator. Repeat as nessesary ,you may have to use some degreaser or mild diluted washing up liquid to clear dried on residue. But be careful to not damage the fins of the radiator as they are more delicate than you realise.