Mary wanted an opening in the front of the Darning Foot I didn’t modify the last time around, so I grabbed it in a machinist’s vise, grabbed that in the bench vise, and freehanded a Dremel slitting saw:
A bit of file work and it looks pretty good, although neither of us like the blurred-from-the-factory red lines:
This one retains the pin that lifts it as the needle rises, so it’s a hopping foot.
The Smell of Molten Projects in the Morning 
Those red smudges are distracting and unattractive, but most solvents that would remove them would probably also turn the foot to goo. You could probably abrade them off with a file or sandpaper, but it would have a matte finish that rather ruins the utility of a clear foot, so you’d have fiddly plastic polishing job ahead of you. 3D printing is out too. I’m guessing you already thought of all this and decided it’s not worth pursuing, unless Mary insists on it, in which case it’s probably time to start looking for the secret cheap darning foot cache.
I have no idea what ink they used for those marks, but it’s definitely Good Stuff. We wish they’d paid a bit more attention to fit-and-finish, as transparency of that foot is absolutely critical for what Mary’s doing now; testing various solvents on the scrap piece convinced me that anything would be worse than what we have now, so she’s living with the smudges.
Suffice it to say that I offered a variety of presser foot improvements in ascending order of crackpottery, none of which survived the initial proposal… [grin]
Or the red markings could become a “feature” to make the opening more visible, or alignment markers, or …
In which case painting over the red with a metallic pen, could be considered “refurbishing”.
(I am only half joking. I got a used Dell laptop, The metal case had been “refurbished” by gluing a metal-looking plastic foil on top.)
This is the spot where the audience begins chanting “Don’t break the seal! Don’t peel back the covering! Don’t look underneath!”