Oh man, six months of cardboard coffee, that hurts to hear. At least you know now, right? I did something kinda close where I never washed out my carafe and just kept pouring fresh stuff on top of old oils. Took me way too long to figure out why every cup had this gross stale note in the back. The hot rinse thing makes so much sense too, heat pulls the paper taste out instead of pushing it into the coffee. Glad your buddy caught it before you hit a full year.

Trapping solvent, yeah that Daytona guy had a point, glad it worked out!

Did you have to adjust your wrist angle at all when you switched to the corner method? I tried it a couple times and kept digging in on the sides, probably going too fast. Also curious how you handle the bald line, since guards kind of hide that area for you. Maybe it just takes more reps before the muscle memory kicks in.