You didn't do anything wrong. You just didn't do the job completely. POR15 seals the rust, it doesn't remove it. You need to pull the inner door panel and attack the rust where it begins. Scrape, wire brush and treat the rust on both sides of the door. You can use Naval Jelly (phosphoric acid) to remove the rust and there are numerous rust converters on the market to use next to convert the remaining rust. THEN you can seal it with POR15.

POR's "Metal Ready" works good also but you have to get complete coverage on the rusty areas whichever method you use.

Of course, FIRST you need to find where the water is entering. The usual suspect is the window seal.

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