Mashed Potato No-No's — What Makes 'Em Gluey, Watery, or Lumpy
Here are some troubleshooting tips for common mashed potato problems:
Lumpy mashed potatoes can be avoided by fully cooking the potatoes, so they mash very easily with a potato masher or stiff wire whip.
Leaving the potatoes in their cooking water after they're done may result in watery, washed-out tasting mashed potatoes. Most chefs drain them well after cooking, put them back in the pot, and place the pot over a low burner to cause excess water to steam off.
Too much mashing, or using the wrong (i.e., very starchy, like an Idaho russet) type of potato can result in gummy, gluey consistency.