I deyellowed my Mac Portable's spacebar using this:
• 3% H2O2 (normal drug store kind)
• 1 teaspoon Oxiclean (boosts the H2O2 slightly)
• Small transparent plastic container.
• SaranWrap (as a cover to prevent evaporation)
• Aluminum Foil (place shiny side up, under the container)
• Two angled...
Below are photos of my 5126 Macintosh Portable's Keyboard with ALPS ivory switches. (Closeup photos below show more yellow than reality. They are ivory.)
Note the SQAURE PADS on either side under the spacebar. The ends of that metal support bar hit the pad rather than smack the metal frame...