Preheat the oven to 350 degrees F. Coat a baking sheet with nonstick cooking spray.
Combine the spinach and artichoke hearts in a large clean kitchen towel and wring out any excess moisture. Transfer to a large bowl.
Add the cream cheese, Parmesan, garlic powder, a pinch of salt and a few grinds of pepper. Use a handheld mixer to beat until combined, about 2 minutes.
Unroll the sheets of pizza dough and stack them. Use a rolling pin to roll the dough out to a 36-inch-by-5-inch plank. Spread the spinach dip in an even line down the center of the dough. Tear each piece of string cheese lengthwise into 4 strips ; line up the strips along the top and bottom of the line of spinach dip, creating a border of cheese around the dip. Fold the dough in half lengthwise. Pinch the edges together in the center to seal in the filling. Use your hands to roll and stretch the dough into a 5-foot rope.
Form the rope into a giant pretzel shape and place on the prepared baking sheet. Bake until the dough is almost completely cooked through and golden brown all over, about 30 minutes.
Dissolve the baking soda in 1/2 cup hot water. Brush the pretzel all over with the soda water and continue baking until dark golden brown and crisp, about 10 minutes more. Brush with the butter and sprinkle with salt. Let cool 5 minutes, then serve warm.
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