Today I had gotten a hold of some GMK keysets, and they were stored inside plastic bags with printed logos like you get from the grocery market. To my horror, many of the keys were stained quite badly with the red printing from the plastic bags. First I tried dishwashing soap and water, rubbing with my fingers and also scrubbing with a toothbrush. The ink just smeared around, and a lot of red and pink was still all over the white caps.
Next I tried using my ultrasonic cleaner (around $40 from Amazon) with some laundry detergent and Oxiclean, but I really didn't have much hope since the stains seemed to be quite set in. To my dismay, after running the ultrasonic for more than half an hour, all the stains were still there with no signs of improvement. I had pretty much lost hope and was next going to try using Magic Eraser to scrape the stains (and also the keycap texturing) off.
However, I idly tried rubbing one of the stains with my finger, and to my surprise, the red dislodged itself and came out! After lots more rubbing, the keys are all clean now with no signs of the red ink. I'm guessing that GMK ABS must be porous, and the ultrasonic cleaner and Oxiclean helped loosen up the ink?
I am quite happy and complete amazeballs right now. All hail the ultrasonic cleaner!