I am also working on this subject and until now I tried to use a pen that says 0.1mm and 0.05mm. In practice I think I get 0.3mm or 0.4mm.
This pen uses pigment ink, and I am put the PCB on oven to dry very well the ink and it seems to resist very well to water... I believe it will work with etch acid. I were trying before to print PCBs with Epson printer and with Epson DuraBrite pigment ink, people is reporting that ink resists to etch acid. - User:Casainho
Casainho, try the Red_Color_Staedtler_Lumocolor_Permanent_Marker_318-2 --Sebastien Bailard 21:43, 23 July 2010 (UTC)