How do you clean fountain pens that were filled with waterproof ink?

Fountain pens running waterproof ink should be cleaned every eight to fourteen days unless they are in daily use. If the ink has nevertheless dried, cleaning begins with a water bath of about six days, possibly with mild soap solution, in which the first ink pieces dissolve. Then the pen, or at least the nib unit, can be cleaned further in an ultrasonic bath; for cartridge pens, additionally tape-head spray from audio retailers, sprayed through the unit, helps to dissolve waterproof ink residues. Writing instruments of celluloid, ebonite or hard rubber must on no account go into ultrasonic, while materials from Pelikan and Montblanc are relatively insensitive there; gold plating already attacked on clip or nib can, however, dissolve further in the ultrasonic bath. Soiled gold nibs can also be cleaned with a damp silver-cleaning cloth, with the cloth being damp so it does not scratch. If nothing helps, the pen should be handed over to a repairer for disassembly.