Tesco CD Marker pens [EAN 500-0462-838307: "4CD/DVD Marker pens] are
typical of the type of permanent marker concerned.

These apparently dry out if the top of left off for only a short
time. Even with the top on storage life seems to be short.

Pens with water soluble ink can often be revived by just wetting the
tip. Is there any way of reviving permanent pens (which presumably use
a spirit solvent)?

TIA