Hi Martin,
I have been struggling to hone my bow pen, but recent saw a YouTube by @MikeTrice that helped me to at least get it working. So will give that a go
I also tried Rotring drawing pens - I think they are black lines you can see in the photo, which are okay.
Halfords do a paint pack (damask red) with a fine tip applicator for stone chips and scratches - see below. But the paint flows too quickly for this application
Otherwise, I used a very fine (000) brush, but it is really slow and keeps drying out so has to be cleaned after a few minutes.
Where the mouldings are crisp it is possible to get okay results. I shall keep persevering!