Bring a large pot of water to a boil over high heat. Add the potatoes and cook until tender, 15 to 20 minutes. Drain well.
Rice the potatoes into a large bowl. Add the evaporated milk, oil, sugar and salt and mix well. Let cool, then cover and refrigerate for a few hours and up to overnight.
When ready to make the lefse, add 2 1/2 cups flour to the potato mixture and mix well. The dough should be sticky and hold together, but not so sticky it's impossible to work with; if necessary, add the remaining 1/2 cup flour. Divide the dough into 2 logs. Cut each log into 9 or 10 pieces and shape into small balls. Work with one dough ball at a time and keep the rest in the fridge.
Heat a grill pan over medium-high heat (or a lefse grill to 400 degrees F). Generously dust a work surface with flour and flour a rolling pin. Roll one dough ball in flour, then use the heel of your hand to press it into a thick disk. Roll the disc into a circle about 1/4 inch thick, lifting and flipping frequently so it doesn't stick; use more flour as needed.
Use a large offset spatula to transfer the lefse to the grill pan (or lefse grill). Cook until the lefse is steaming and small bubbles appear on the uncooked side, about 1 minute. Flip and cook for another 1 minute on the second side. Transfer the lefse to a clean dish towel and cover with another. Repeat, stacking the lefse atop one another between the dish towels.