Bleach is the only thing that actually kills cooler mildew.

The best way to fix a stinky, mildewed cooler is to wash it with soap, hit it with a dilute bleach solution, and then dry it completely before storing.

Plain soap and water will remove dirt, but it won’t kill mildew spores. That’s why the smell comes back after a week. Mix about one tablespoon of bleach per gallon of water, wipe down every interior surface, and let it sit for 5–10 minutes. Then rinse thoroughly so you don’t taste bleach in your next batch of beers.

If there’s still a faint odor after that, it’s usually trapped in the rubber gasket or drain. Wipe those with vinegar, then leave an open box of baking soda inside for a day. The real secret,