a while ago someone or something hit my 300SD in the rear upper rught corner of the car. I was able to easily bang the dent out from inside the trunk but now, there is rust where the dent was. I have touch up paint from MB but I need to kill the rust first. What should I do?

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W126 1983 300SD 286,000 miles and ticking
Baby blue exterior Grey MB tex
Recent work:
Replaced air cleaner mounting brackets and heat shields
Replaced alternator, fan and power steering belts
Replaced positive battery terminal
Replaced negative battery terminal and cord
New Duralast Battery

My car needs work.