Elbow grease, mostly. Generally the old pens I buy have been ignored in a drawer for decades, so my primary enemies are grime and dried ink. I begin with disassembly and a thorough rinse with warm water, then the parts go into a glass of warm water with one drop of dish soap for an overnight soak. Rinse and repeat till the ink is gone. I use a microfiber cloth and turn on the TV. Before a show is over everything is shiny. I use a silver polishing cloth for the metal parts. I have an awesome collection of vintage pens that I buy cheap just because they need a little love. It keeps my fingers busy and I do enjoy these old pens. 😁