Rinse the sardines under cold water and pat them completely dry inside and out with paper towels. Dry sardines will crisp much better.
In a small bowl, mix together the kosher salt, black pepper, smoked paprika, garlic powder, cumin, and cayenne pepper.
Drizzle the olive oil over the sardines and rub it into the skin. Sprinkle the spice mixture evenly over all sides of each fish, pressing gently so it adheres.
Preheat the air fryer to 400 degrees F for 3 minutes.
Arrange the sardines in a single layer in the air fryer basket. Depending on the size of your air fryer, you may need to work in two batches. Do not stack the fish.
Air fry at 400 degrees F for 8 to 10 minutes, carefully flipping once halfway through, until the skin is crispy and golden and the flesh is opaque and flakes easily with a fork.
Transfer to a serving plate, finish with a pinch of flaky sea salt, and serve immediately with lemon wedges for squeezing.