I never had any luck with any type of cleaner, I just pull them and have them sandblasted with coarse sand.
Glassbeading always seem to leave a dull shine where coarse sand would bring it back to the out of the box appearance.
Of course I just got my valve covers back from the blaster and the first person to come over had to say "those look nice and" put his hands on a cover, leaving a nice shadow that even carb spray can't remove.