First, find and repair the leak.

Rear glass seal, boot/trunk lid seal and antennae mount are the usual suspects. You are wasting your time trying to "band-aid" the problem.

Second, identify the extent of the rust. Flash light, mirror, boroscope, what-have-you.

Third, decide on your plan of remedy.

Don't just willy-nilly pour some in "miracle" sealant expecting it to solve the problem. 90% of the time it aggravates the problem by blocking drainage passages.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now