This is what I would do. First get a bottle brush (usually available in baby aisle or houseware aisle): 1. Fill the flask with water containing some dishwash liquid. Leave overnight, empty, rinse. 2. Fill the flask with diluted vinegar, leave overnight. 3. Empty out some of the vinegar, use the bottle brush to scrub the flask as best as possible. Empty, rinse. Try these steps 1-3 (a couple of times if necessary).