Slice the sweet potatoes lengthwise into approximately 1/2-inch slices, then cut each slice into thick fries (1/2 to 3/4 inches). Place the fries in a bowl of water at room temperature for 2-4 hours.
When ready to bake the fries, preheat the oven to 425F. Drain the potatoes and blot dry with a towel. (They do not have to be completely dry, so don’t spend a lot of time on this.) Place the arrowroot starch, salt, paprika, and pepper in a paper or plastic bag that can hold all the sliced potatoes. Add the potatoes to the bag, hold it closed, and shake vigorously to coat the fries evenly.
Remove the fries from the bag to a bowl (or you could use your baking pan), and pour a few glugs of oil over them. If there is a little remaining seasoning in the bag, you can add this to the bowl also (the salt seems to settle out to the bottom). Toss in the oil until evenly coated. They should not be dripping with oil, but they need a good coating to get crispy.
Line a large baking tray with foil, and place the fries on a rack over the tray. Do not crowd them too much or they will not crisp up. If you have a convection fan in your oven, use it now! Bake for around 20 minutes, until the fries are lightly browned and crisp to your liking. Serve them hot!