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In particular, I've had white blocks that have yellowed in places. Can the colour consistency be restored? Or at least improved?

I'd be more interested in people's actual experiences rather than simple suggestions.

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Has anyone tried using Vanish Oxi Action (or other oxygen based bleaches)? –  Umber Ferrule Oct 27 '11 at 13:13

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When I first got my kids some Lego I did it by bulk buying 25 kilo bags of used bricks off the Internet. Now these bricks had obviously been in outdoor sandpits, chewed by dogs, hidden under carpets - you name it.

We successfully restored almost all of them by by washing them in a bath full of a Milton solution, scrubbing each one with an old toothbrush and then leaving them to dry on a towel.

This cleared up some yellowing that we had thought was sun damage and fading, however there were some (especially some black pieces) that just looked off-grey and no scrubbing sorted them.

I would suggest trying this method, but if it is sun damage that is more than the outer surface I think you might not have any success.

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For those of us not in the UK, Milton Sterilising Fluid is a weak solution of bleach (sodium hypochlorite). Similar products in the US might be Clorox Anywhere Hard Surface or Fantastik Cleaner with Bleach. –  Dori Oct 26 '11 at 3:50
    
I was about to answer that's it's usually hopeless, so it's good to see there's a solution, even if it's partial. –  Joubarc Oct 31 '11 at 5:27
    
I wonder how the plastic reacts with this solution. Some products get into the plastic, reacts with it and can make it more brittle. I would be careful before using anything else than natural soap and water. –  pcantin Oct 31 '11 at 14:49
    
No reaction at all - admittedly, Milton is the bleach of choice in the UK for cleaning kids toys etc., so it is pretty mild. –  Rory Alsop Oct 31 '11 at 15:38
    
A quick wash in a dilute bleach solution is not likely to harm the integrity of the plastic. A prolonged soak in a strong bleach solution, however, can leave pits in the surface of the ABS plastic and make it brittle. –  62Bricks Mar 13 '12 at 3:03

I have used the Retr0bright method with great success for restoring yellowed bricks. Originally, this solution was hit upon by fans of retro computers wanting to restore old yellowed computer and game consoles to their original color. LEGO fans picked up on it and have had good results.

The solution is essentially hydrogen peroxide with sodium percarbonate added as a catalyst. The bricks are submerged in the solution and placed in UV light (sunlight or under a UV lamp). Depending on the strength of the solution and the light, the process can take several hours to a couple days.

Hydrogen peroxide is readily available, although in the US it is usually sold in a fairly weak 3% solution. Sodium percarbonate is the active ingredient in various "Oxy" detergent boosters.

The Retrobright site has a complete discussion of the chemical reaction that takes place which restores the original bright color. The site owners do not recommend bleach for this purpose. Although a prolonged soaking in bleach solution can remove much of the yellowing, it will also leave pits on the surface of the plastic, causing it to become brittle.

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