The method

After testing many methods, I've finally found a reasonably foolproof method to completely rebrighten small to mid size items, and used another method to obtain reasonable improvements in larger, unwieldy items.

Plastics yellow for a variety of reasons - diffuse sunlight, fluorescent lighting, heat, and just generally as they age. You will never restore plastics to how they were when they originally came out of the mold! That said, you can get pretty close with results that can be indistinguishable to your human meat eyes.

There are two methods in use here - sealing in plastic bags full of peroxide for the smaller items, and what is known as "lightbrighting" for larger items that can't be diassembled. This is as simple as leaving the item outside on a sunny day for several days running. It doesn't make things perfect, but can brighten large items a few shades.

Instructions - sealing method

• You will need: sous vide bags, a heat sealer, liquid Hydrogen Peroxide (H₂O₂)
• Cut a strip of sous vide bag that is significantly longer than the item you're going to brighten. Seal off the bottom, and seal the side so that the item fits with a little bit of play. I recommend sealing again 1cm away to prevent leakage.
• Put the item in the pouch you've just made and fill with peroxide. You can mix with water if the item is larger.
• Seal the top - you want to pull the pouch up to the sealer so that there's as little air as possible. Then seal again 1cm away to prevent leakage.
• Throw the whole thing outside on a sunny day with some tin foil underneath (to reflect sunlight back at the non-exposed side)

Once you're done, it should look like this - click for larger pictures!

Leave outside for 1-2 days in bad sun, or a few hours on a good sunny day. Because they're sealed, you barely have to check on them! If they aren't back to the original color, repeat the process.