First it should be cautiously tried to turn the cone; if this does not work, professional repair is advisable (e.g. via Maxpens.de or the repair section on fountainpen.de). Important is to turn only at the cone, not push or pull. If the piston can be moved, the pen can be filled with distilled water; on rare 1950s models caution is warranted, since celluloid softens with prolonged water contact (max. two to three days soaking). The cork mostly has to be renewed; occasionally, however, it swells back. For polishing Wenol is suitable; dried ink in the sight window can often be dissolved with patience and ultrasonic.