Baked sweet potato french fries (chips) too soft, how to crispen

Here is an excellent recipe by the guys over at Cooking Issues (and also a follow up report) with almost everything you'd ever want to know about chips. That first recipe is supposedly good even after they've cooled off:

Our standard fries are good even when cold.

Another option is to cook a lot of chips but stop after their first frying and freeze them; when you want to eat them, simply put the frozen chips into the fryer for the second frying. This also has added benefits:

Freezing acts like partial dehydration. When the frozen fries are finished, they liberate water freely, leading to rapid dehydration and good crust formation with a porous interior. Pre-frozen fries are crunchier than fresh and stay crisper longer after they are fried, but they tend toward hollow fry.

There are a couple tricks I've learned from Cook's Illustrated about making oven fries:

1. Soak your cut potatoes in warm water to rinse off excess surface starch. Drain them and dry them very well (I use a salad spinner and paper towels).

2. Use a heavy duty sheet pan on the bottom rack of a hot oven to focus the heat on crisping the bottoms of the potatoes.

3. Oil your sheet pan well, and sprinkle salt on it. This will act kind of like ball bearings under the potatoes to prevent them from sticking too hard.

4. For the first 5-10 minutes, tightly wrap the sheet pan (with the potatoes) with aluminum foil. This will essentially steam the potatoes and help get a really nice creamy interior. After that, remove the foil and flip the potatoes halfway through. Make sure you use a thin metal spatula when you flip them.

These techniques work well for potatoes in general, and really do turn out some fantastic spuds.