For fresh made russet potatoes, they often will turn gummy if over mixed. If that’s not the issue, an easy fix for watery potatoes is to break out the instant dehydrated potatoes and mix them in a tablespoon at a time, into the wet potatoes. This usually will thicken them up. You can also leave the potatoes on the stove longer, being careful to stir frequently in order to cook some of the moisture out or place them in the oven at 325 degrees F for 10 to 15 minutes to "dry" out the mixture.

For more information click here: