If you want perfectly crispy roasted potatoes, parboil them with baking soda
I tried this last sunday. Before putting the potatoes in the oven, boil them for 10 minutes in water with half a teaspoon of baking soda. It breaks down the outside of the potato into this starchy "mush" that, once roasted with oil, turns into the thickest, crunchiest crust I’ve ever seen. I’m never roasting them the old way again.