Deyellowing old computer equipment: Retrobright

The original method using guantham gum is rather complicated with some uncommon ingredients. I used the method written up elsewhere, with corn starch powder to make a gel. But it can be done even more simply, by just adding 2 compunds. Needed:

I mostly use only 3% hydrogen peroxide + vanish oxi action gel. I just mix it in near similar amounts but diluting with water still works well, so the exact relative amounts are not so important. It's quite thin but workable. Diluting further with water still works very well on gray plastics that I tried.

The results vary quite a lot in how well it works (how much it deyellows) per application of it on the plastic, per type of plastic. Some plastics deyellow/debrown almost instantly. esp. gray plastics as for example the Eizo 8030M monitor for my BBC micro. For this 2 applications in bright sunlight are enough and it's really back to original colour (as can be seen from small damaged bits where you can see thus below the surface). Beeb beige is far more difficult, multiple applications over multiple days, and then still they are not quite as new. Severe yellowing quickly gets removed, but getting to the original colour takes a long time.

You can also try thin see-through clingy film (for food normally) to wrap the objects that you want to deyellow (to make the gel not dry out too soon). I'm not sure it's needed/useful, it didn't seem that effective so far, but I need some yellowed gray plastic to try out to be sure if it's effective.

BBC micro keyboard caps deyellow quite quickly, 1 or 2 applications in bright sun for a while (I just leave it for a few hours) and the letters are sparkly white...

On the gray plastics it's easily seen that the brown stuff comes off but sticks on top if you don't wash it away which gives streaks. Just apply another time. I need to do that with the Eizo 8030M, which was really brown, I thought it would become beige, but instead its original colour is gray! Uneven deyellowing is thus not a problem, just try it again.