Did you use anti-freeze/coolant??
Looking at the gunk on the pic I would use a radiator cleaner as per the recommended instructions on the bottle and then flush it out with clean water, and use a GOOD QUALITY (expensive) antifreeze/coolant diluted to the specific instructions of that product. Too much antifreeze will damage the motor, and too little will just make it an expensive bit of inefficiency.
A clean radiator give piece of mind !! Mine after 4 odd years was still clear and the blue colour ( same as when I poured it in ) and when looking down the radiator cap opening, the metal still looks shiny and clean.
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