Answered by Sidi Mostafa Azzam
I would like to write with a traditional ink pen, not a biro or roller ball, but I was wondering how it affects wudu, since it always leaves marks and damages the hands to scrub it off. I think it stains them like henna rather than biro coating the surface of the skin. Is this right?
Says The Immaculate Raiment: “It is necessary for ablution and bathing that unfilthified, unused water flow over all that must be washed.” The point is, water must flow over the spot. Therefore, anything that prevents the water from flowing over the spot prevents the validity of the ablution; anything that does not, does not.
As the fuqaha indicate, only a body can prevent water from flowing over the spot; a stain cannot. You can distinguish a body from a stain in that a body can be scraped off; with a stain, there is nothing to scrape off. You said, “it always leaves marks”: a mark is not a problem; a body is. On a practical level, just look at it: a body is three-dimensional; a stain or mark is two-dimensional. Is there actually a substance sitting on top of the skin or merely a coloring of the skin?
Regarding ink and henna in particular, the fuqaha mention that ink and henna themselves (i.e. their paste-like substances) are a body, whereas the stain they leave behind is not.
The upshot is that the stain left behind by ink (whether from a fountain pen, a ballpoint pen, or anything else) is not a body and therefore does not prevent the validity of ablution, since it does not prevent the water from flowing over. As for if the ink itself (i.e. the thick liquid) is sitting on the part washed, this does prevent the validity of ablution, since it prevents the water from flowing over.
And Allah knows best.