Big Lee did a small test of a Uni Pin Fine Line Pen, testing to see how waterproof and smear free it was under different conditions. He then used them to draw tiny symbols on some 6mm Carthaginians, with some very nice results.
The pens in question are apparently not sold in the US, and imported are around $4 each (on Amazon anyway), so I thought it might be useful to do a similar test with some pens which ARE available here, the Sakura Pigma Micron Pens, which aside from being cheaper and more readily available, also seem to come in various sizes and colors.
So, following Lee's example, here are the four tests.
1. Gesso base, Black Micron 0.2mm written over brushed on Flat sealer (craft store)
2. Gesso base, Black Micron 0.2mm written over gloss (Future)
3. Gesso base, Black Micron 0.2mm written under brushed on Flat sealer (craft store)
4. Gesso base, Black Micron 0.2mm written under brushed on gloss (Future)
Writing on the bare Gesso was very easy, and there was no smearing or other difficulties. Brushing the flat and the gloss over the top seemed to smear the ink, although I did it right away, so it is possible that letting it sit for 24 hours would reduce this. Interestingly the gloss seems to have pulled up the ink more than smeared it, so you have sort of a shadow effect as if you applied a wash, which is actually what I noticed when I used these pens way back when on some TOS destroyers, although at the time I attributed it to the wash I used. Writing on top of the flat and gloss was harder, and although it is a bit hard to make out due to glare, the writing is a bit more sloppy due to the tip slipping as I was writing.
I think that ideally you would put this on as close to bare paint as you could manage, and then spray a flat sealer over the top. For some paint jobs this may mean adding more paint after you have "finished" a paint job, but I think it would give the best look.