For mashed potatoes I boiled potatoes with skins on when I usually peel & cut them before boiling. (I must have thought I was making potato salad instead of mashed potatoes.) When they were done I cooled them down so I could handle them. Then peeled them & tried to mash them with a hand masher difficult. Warmed milk & butter, added to potatoes & continued to mash. Potatoes were still stiff. They tasted fine, but I never got the fluffy consistency I usually get when I boil them without skins. Could I have done anything to get the fluffy consistency?
Chances are the interior of the potato was not fully cooked. That’s why we recommend peeling and cutting the potato into even sized chunks, 1 ½ to 2 inch cubes and then boiling. For a drier mashed I’ve had success in baking potatoes with the skin on till 210 degrees F inside, 185 degrees minimum. Then using oven mitts, slice the cooked potato in half vertically, scoop out the insides and mash, turns out extra fluffy as the oven baking has removed a lot of the moisture.
All Things Mashed Potatoes HERE