A server in a Cajun restaurant told me that they used a cornstarch and seltzer batter to coat their sweet potato fries. I tried it once and they came out pretty crispy. He didn’t share the proportion of starch to seltzer so I used this recipe as a guideline http://www.foodnetwork.com/recipes/je...
I also soaked the fries in water for about 30 minutes after reading several recipes online that recommended doing so.
I make roasted sweet potato in the oven too. I cut the sweet potato into 1" cubes. Try think lightly oiled, at a higher temperature ~375F for about 35-40 minutes in an uncrowded oven. If there are too many things in the oven they just steam instead of crisping.
Perhaps seasoning some flour or cornstarch would get that light coating on commercial fries like suggested below.
We make them all the time and try different variations. Just slice sweet potatoes to as thick or thin as you like them. Pat them dry and place into a large bowl. Make a mixture of olive and/or grapeseed oil. Add fresh cracked pepper (white & black), kosher or celtic salt. We've also used chile powder, cayenne pepper... the sweet/spicy/salt end result is great. Toss the oil mixture with the sweet potatoes. Bake at 425 until browned, time depends on thickness of slices. You can turn the slices 1/2 way through baking, but I've found that it works almost just as well without. Enjoy:)
IME these are usually processed sysco garbage these days, crispy and tasty as they may be. I don't think they're actually cut pieces of whole potato anymore, even, but some conglomeration of something that then seems to have been coated in something else.
Anyway. I have had decent luck making real ones at home, but you need to bust out the hot oil.
Following the twice-fried (aka blanching in oil) technique that works for well for crispy regular fries would be my first suggestion. You may also find that dusting them in cornstarch or Wondra or something either pre-fry or after the first fry will help to make them crispier.
I would also suggest cutting them quite thin.