Article: Put the baking soda in a small a bowl and stir in a little bit of water. The consistency should be a thick paste. If the mixture is too thick, add a little more water to thin it out. If you put too much water and the mixture is too thin, add a little more baking soda to thicken it. Dip a bottle brush into the paste, making sure to coat the bristles heavily. Use the brush to scrub the inside of your flask. Target areas that are badly stained by using small, circular motions. Repeat this step as needed. It may take a few goings over to completely remove the stain, so don't worry if the stain doesn't come out on the first go. Fill your flask with warm tap water. Use the bottle brush along the insides of the flask to loosen up the baking soda paste. Swirl the water around the bottle a few times and then pour it out.  Try filling the bottle up about halfway with water, putting on the lid, and gently shaking the bottle up and down. Pour the water out and run fresh water into the bottle. The agitation will help clear out excess residue. Once there is no baking soda residue remaining on the inside of the flask, continue to run the warm water into the flask, swirl it around, then pour it out. Repeat this step 2 or 3 times, or until the water runs clear. Place the bottle in a dish drying rack, or try propping it an angle against your kitchen wall or the side of the sink. Just make sure it has proper air circulation to prevent bacteria from growing. To help with timing, try washing your Hydro Flask in the evening to allow it to dry overnight and be ready to use the next day.
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Mix 2-3 tbsp (30-45g) of baking soda with warm water to form a paste. Scrub the inside of your Hydro Flask with the paste. Rinse your Hydro Flask thoroughly with warm water. Dry your Hydro Flask by allowing it to sit upside down.