You should wash and scrub your potatoes thoroughly, and then either grate them over a bowl or cut them up and put them in a blender. There is no need to remove the peels. If you’re using a blender, give them a whirl until they’ve become quite liquid. If they seem too dry, you can add a small amount of water. Pour out the mixture, soak cotton balls thoroughly in it, and apply to sunburn. An easier perhaps more effective method would be to soak gauze and lay it over the burn.