Delta Faucets are famous for using recessed aerators where you use a plastic key that meshes with the aerator and allows it to be unscrewed. These aerators can get plugged with time. However if calcium builds up in the threads it’s very difficult to unscrew. The quick solution is to drill a hole in it and the force a screwdriver in it and try rotate it counter clockwise. It that doesn’t work use a tapered paring knife and jam it in. It will cut a grove in the plastic and allow the knife to rotate it. I actually tried a left hand bit but it didn’t unscrew it. The toughest part is to find a low clearance drill that has room to fit in the sink. Here are a few shots.
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