Most people would use some 0000 steel wool with a cleaner like dish soap and scrub it off.
Another method is to use a rust removing chemical like evaporust, but you want to be absolutely certain that it is only surface rust on a welded part of the kettle like the handle. If there is rust through the chemical could eat so much that it makes the weld weak or gone. Or If there is rust under the porceline it could cause chipping.
After you remove the rust you can use touch up paint if it is black. If it is another color you can try paint but it will eventually turn black with heat. Many here would just use some type of cooking oil and heat to season the bare metal.
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