How do you clean your radiators?

My computer is in a very awkward position jammed between a 90 kg cockpit and a 30 kg subwoofer. I have the radiator in an external case at the back and obviously neglected cleaning it for a bit to long. I was kind of chocked seeing the amount of dust being collected. Not a chance to get it out with compressed air had to use a brush to get as much as possible out of it. But there is still a ton of dust jammed in it.

My cooling is way overkill anyway so don´t hurt much but when my video card breaks I am planning to clean up my watercooling loop more thoroughly.

Can you put it into liquid and clean it that way?

yeah, just be careful of rust.