If the surface you are writing on is firm you should have no trouble with
clogging nibs on Faber Castell or Rotring Rapidographs. Slightly chalky
shells are a problem, and should be consolidated with polyvinylbutyral
(PVB) plastic dissolved in highish-grade ethanol or acetone. Hoechst
Mowital B60HH is probably best; also sold under brand name Butvar. These
should be available from scientific suppliers.
Andrew G