Most of retro computers were manufactured in 80s and 90s. After all these years plastic parts often turns yellow, or even brown. Thats mostly caused by exposing it to UV light like keeping it in sunlit place etc. But even without UV light plastic will degrade over years. Except for changing color it can also become brittle. That said one may wonder… How is it possible that you can still buy old computer and it looks just like new? Well, basically it is possible because someone probably refurbished the computer or it was kept in environment which allowed it to be preserved in excellent condition.

So how old computer is refurbished? There are many ways. For example I start by carefuly taking computer apart to separate electronic, plastic and metal parts. Each of these needs different treatment. Electronics may be simply dusted off with soft brush. You can also use special fluids for cleaning electronic parts and PCBs. Most common one is isopropylic alcohol. But thats not the topic we will discuss. Also metal parts restoraion will not be covered in this text. Lets just say you may need to clean metal elements like board shielding or RF modulator boxes from rust and protect them against it in the future. Today we will focus on deyellowing plastics.

Deyellowing plastic and restoring it to original color is often called retrobrighting. There are few ways to accomplish this task. I’m using most common one. I start off with washing all plastic parts with warm water, soap and soft or medium brush. I also remove all unwanted stickers, pen marks etc. Plastic needs to be as clean as possible. Now comes the magic… You will need hydrogen peroxide cream sometimes known as oxidant or bleach. All plastic elements must be covered with it. Don’t do that with without gloves (you should wear them when dealing with any chemistry). Use soft brush and evenly distribiute the cream on entire plastic surface (1-2 mm layer is fine). Now wrap it tightly in transparent foil, put it out in sunlight for few hours and you are done… Maybe :-) You can also use clear liquid hydrogen peroxide and just soak the plastic in it and expose it to sunlight. It is believed to be a better and safer method than cream (less risk of bad results). While this is okay with smaller components like key caps, it is problematic with cases (you’ll need a clear plastic box large enough for the case to fit).

Depending on plastic type, condition, environment, cream layer, stable tempereature etc you may get various results. From perfect restoration, through partial restoration to even complete failure. Some plastics won’t get deyellowed, some will but only partially, leaving you with either still yellowed elements or ugly blobs and splotches of yellowing. Some plastics will be damaged by the process, especially dark ones may become discolored. Also even lighting is important. You must either manually rotate you elements from time to time or invest in simple spinner/turntable that will keep rotating your plastics for you. Otherwise elements in shadow or with less light will remain more yellowed than the rest. On a side note, UV light isn’t necessary. Warm environment will do as well, but deyellowing will take much longer, like days instead of hours. Some people prefer this method, especially because UV light hence sunlight will make plastic brittle.

Now lets talk about hydrogen peroxide cream. It is available in few variants, usually 3%, 6%, 9% and 12%, at least where I live. You may need to check product specs to find out. Which one should you use? That depends. But lets try to find out the difference by performing small retrobrighting. I found heavily yellowed PC keyboard in my attic and used its keys as samples for our tests. 0 key will be used as our yellowed sample, we won’t do anything to it. 3, 6, 9 and 1+2 keys will be treated with 3%, 6%, 9% and 12% cream. So lets see how keys look after spending 5 hours in direct sunlight wrapped in transparent foil (except for 0 key). The photos may not show exact results so I wrote a few words based on my visual inpections and comparison.

Obviously, the stronger the cream is, the stronger retrobrighting was achieved. Lets compare each reasult against our sample key.

The 3% cream did make a big change, but the key is still yellowish. Not really refurbished.

The 6% cream gave much better results, yet still it looks a bit yellow. Not really refurbished.

The 9% cream gave very good results, key is still slightly yellowed, but it isn’t that much noticeable unless you compare it with 12% results.

The 12% cream gave the best results, no surprise here. Keys have returned to their natural color. I tried retrobrighting them for another 5 hours on the next day but that really didn’t change anything.

What is the conclusion? Its obvious – the stronger the cream is, the stronger and thus faster deyellowing is. It is worth noting that even 3% cream will eventualy achieve same results as 12%, it just needs more time. But remember – more time in UV light means more brittle plastic. If you are not in a hurry you may as well store your creamed and wrapped plastics in warm room and forget about it for few days, a week or even more depending how strong yellowing is and how strong cream was used. The fastest way to retrobright on the other hand is to use stronger cream and put your plastics in sunlight. Depending on yellowing level, sunlight strength and environment tempearture it may take anything between 30 minutes to few hours to complete the process. Be aware that using too strong cream and/or keeping it too long may cause random and often ugly discoloration on some plastics (see phtoto below). There is no rule here which plastic will be prone to discoloration. For example some Amiga cases will get discolored and some not. So it is worth checking from time to time to see how it is going, if you need to add more cream and if foil is tightly sealed/wrapped so cream won’t dry off.

Few rules that I follow when doing retrobrighting:

  • always clean your plastics with soap and warm water before retrobrighting
  • wear gloves or avoid contact of cream with your skin, wash it out as soon as possible if it got on your skin
  • be carefull not to get the cream on your clothes, you can damage or discolor them
  • when doing new kind of plastic/element try it first on some non-visible parts
  • the stronger yellowing is, the stronger cream should be used (but not too strong)
  • use as even layer of cream as possible
  • try not to exceed 2-3 hours in direct sunlight, especially when its hot
  • keep rotating your plastics and inspect the process from time to time, add more cream if necessary
  • if you think it is done, stop the process (it is better to not deyellow completly than to permanently discolor your plastics)
  • avoid retrobrighting non-white plastics, it usually does more harm than good
  • when done always give your plastics a very good clean with soap and warm water, be sure to remove all of the cream
  • if there are still some yellowing, try again, change cream strength if necessary

Ok. So now you have some basics. Now lets take a look what retrobrighting can do for real retro computer. Here is an Atari 800XL with nearly brown top part of the case which should be nice, bright and a bit creamy. It took 4 hours in sunlight using 12% cream to get the top deyellowed. Unfortunately both left and right sides were still a bit yellowish. There were also 3 yellow blobs left between Atari 800XL badge and cartidge slot. So case was put out in sunlight again, this time with 9% cream and for another 4 hours. The result is excellent. This computer is also good example why retrobrighting of dark plastics isn’t recommended. Check brown plastic betweeen function keys “start” and “select” and rest of the keyboard on the second photo. See that grayish blob? Ugly isn’t it? It can be masked off with some chemistry dedicated for keeping plastics nice and shiny but it will never look as new again.