For the rough work, a fine sharpening stone is suitable; afterwards, however, the tipping must necessarily be smoothed with ever finer abrasive paper down to 10,000 grit. Suitable fine-abrasive sheets, e.g. as round pads, are available via specialised providers like David Broadwell. Caution with iridium: too much removal makes the nib unusable; after smoothing, the ink flow can if necessary be optimised via re-adjustment of the nib unit.