In case you have overmixed your potatoes or used the wrong ratios and now need to thicken your mashed potatoes, there is a solution. You can either continue to cook the watery mash in a pot, where the heat will help the excess moisture to evaporate. Although it is a little risky move as you can burn your mash and make the potatoes gummy instead. Another option with very watery potatoes is to thicken them with either flour, cornstarch, or powdered milk. Start with 1 teaspoon at a time, stirring it into the potatoes mixture.