How would you experts go about fixing this rust spot before it gets worse? It is the only place in the car I can find any signs of rust. I was thinking it would have to go to a body shop but maybe there is a DIY fix?

Here is a pic:

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2004 E320 4matic
2000 E320 wagon
-- in the past---
1992 190E 2.3 5-speed! (European Delivery Baby)
2000 ML320
1978 240D - 340k mi