You must clean the magnetic gap in a compression driver before installing a diaphragm and in a speaker before replacing the voice coil. You must make sure that no dirt, metal filings or pieces of old vc windings remain behind that will negatively affect the repair.
Use a 1/4″ piece of shim or thin card stock. Fold a piece of masking tape over it- sticky side out. Pull tight. Insert into gap and move 360º until you have worked around the entire circle. Repeat until the tape remains clean several times. As a final step, visually inspect.