1. Place potatoes in a large pot. Add enough cold water to cover by an inch. Bring to a boil; add 1 tsp of the salt. Cook 10 to 12 minutes, until just tender. Drain; rinse with cool water. Cut into 1-inch pieces.
2. Meanwhile, cook bacon in a skillet over medium heat until crisp, about 8 minutes, turning. Remove to a paper towel.
3. Transfer potatoes to a serving bowl; add celery and scallions. In a small bowl, whisk together mayonnaise, buttermilk, 2 tbsp water, the mustard, sugar, remaining 1/2 tsp salt and the pepper. Crumble bacon over potatoes; toss with dressing. Refrigerate until serving.