It's really only the radiator. I suggest using a 50/50 mixture of boiling water and vinegar. Pour it into the radiator so it's about 1/2 full, then reseal the end caps using the little white plugs that come with it. Then pick up the radiator with oven mitts on (as it will get VERY hot) and shake it quickly. Then after about 30 seconds drain it all into the sink.
You just want to remove any flux and machine oils left over from the manufacturing process.
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Either metal shavings or steel wool debris... Clean it out a couple times.