"RetroBrite" is a name for a process that restores old yellowed plastics to new. Many plastics yellow or darken over time. RetroBriting can reverse this aging, but with a number of caveats that, in my opinion, make it pretty useless. While the process actually does work - it reverses the discoloration - the effect is temporary. After a few months, the plastic will return to its yellowed state. When this happens, you can repeat the treatment, but at some point the chemicals involved are going to start to degrade the plastic.
My first experiment was whitening some yellowed tuner knobs, and it did work. With nothing more than sunshine and hydrogen peroxide, the knobs lightened considerably. That was several years ago, and today the knobs are as yellow as ever.
What causes this yellowing? It is variously attributed to sunlight, oxygen, bromine content, and other causes. While all of these things can contribute to it, none of them are necessary. Some plastics simply turn yellow with age, and nothing will stop it. In my experience, the real culprit is simply bad plastic, and the only real solution is replacement.
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