How does the staining process (for pens) work by the way? I imagine it doesn't simply occur immediately on contact and that there are other factors at play.
The way I think about it, there are a few different sorts of "stains". One is just ink getting caught up in the lubricant of a piston or plunger filler, which is not really a stain as such, but it appears that the ink cannot be washed out completely. Another sort of "stain" is a dry residue that resists rinsing but can be wiped out, which happens a lot with the cellulose-reactive inks, but I see it mostly on the nib. The third is the permanent one: molecules migrate into the plastic itself, which takes some time.