Be sure to keep refrigerated or chilled at all times. If mixed in stainless steamer trays, and you use the stainless lids, these can stack in the refrigerator.
For serving we use our steamers unplugged with a layer of ice dumped in the bottom. The trays of potato salad can be placed in on top of the ice. Additional trays of ice can be placed on top of the potato salad. Kept chilled until right before serving.
Potato Salad at a picnic, or even inside the house, left to get warm. Can have unpleasant and even deadly consequences. Practice good food handling safety. If I see someone else's potato salad [or deviled eggs] and it isn't chilled, not sitting on ice to keep it cold while serving....I pass.