Rice flour works, it's like tempura. Baking soda does help with the crunch. But it's the carbonation in the liquid that really makes the difference. Make two batters, one with tap water and one with seltzer and compare.
OP, tell us about the sweet potato fries? How hot? How long? Tossed in oil and baked?
I find it very hard to get decent sweet potato fries without using a deep fryer. They burn easily, plus they're soft and don't really crisp up. I've tried different thickness cuts, different oven temperatures, par-cooking, different fats, etc. They come out OK, but not crisp without burning. I can make great fries in the oven using yukon gold potatoes, but sweet potatoes are a challenge.