Personally, when my cooling head gets all gunked up, I:
1. Take it off the engine.
2. Remove the glo-plug.
3. Throw it in the kitchen sink with some soapy water and a scrub brush (bottle cleaner style).
4. I let it soak for a bit and then give it a good scrubbing.
5. Once clean, I use a hairdryer to get it nice and dry prior to replacing it on the engine.
Seems like a lot of work, but it works well and keeps the gunk from getting blown into the other parts of my truck. It also allows me the opportunity to check the internals of the engine and apply some ARO to the piston and sleeve.