How To De-Yellow Old Plastic Parts

I’m sure you’ve seen old electronic gear that has gotten yellow over time… old Apple Macintosh computers are famous for this.  This is due to a process called Photodegradation, which is explained in great detail in this paper from researchers at Al-Nahrain University in Baghdad: .  To summarize and greatly over-simplify, ultraviolet light destabilizes the plastic and changes it on a molecular level, resulting in yellowing.  Fortunately, this process is reversible by exposing the plastic to ultraviolet light while in the presence of hydrogen peroxide.


I recently acquired a 1964 Adler Tippa 1 which had very yellowed keys that didn’t match the rest of the machine.  I suspected that the keys matched the machine at one time, but had yellowed over time while the ribbon cover, made from a different plastic, had not.  Unfortunately, the keys on this machine seem very difficult to remove, so I decided to try it with the keys in place.  To prevent water and peroxide from dripping down inside, I used some thin craft foam that I bought as a replacement for felt sound-deadening on the floor of the machine.

keys before retrobrite

I used two strips of foam to isolate a row of keys at a time:  one above the row, the other below it, meeting under the keys in sort of a “V” shape, and held the ends together with a couple of binder clips.

cleaning keycaps with toothbrush

The keys themselves were pretty dirty, so first I gave them a good scrubbing with a damp toothbrush.  These particular keys have a fingerprint-like texture molded into the tops of them, and I’ve found the toothbrush is the easiest way to get the dirt out of all the little ridges.

applying developer to keycaps

For hydrogen peroxide, I used 50 Volume Cream Developer from Sally’s Beauty.  It’s a strong concentration, and the cream formula enables it to be painted on and stick to the keys.  Make sure to wear gloves when using this stuff, and don’t get it in your eyes, you’ll regret it!

ready to cover with plastic wrap

Once all the keys were treated with developer, I covered them with a strip of plastic wrap.  The idea here is just to keep the solution from drying out before the de-yellowing is complete, so the plastic wrap doesn’t have to be completely air-tight.

tucking in plastic wrap

I used a table knife to tuck the plastic wrap under the row of keys.

ready for sunshine

All tucked in and ready for some sunshine!  But first…

cleaning lever knobs with toothbrush

…I wanted to de-yellow these lever knobs as well.  This time I just cut a couple of slits in a piece of craft foam for the knobs to poke through.  Then a quick scrub with the toothbrush…

lever knobs ready for sunshine

…I applied some developer with a q-tip and covered with plastic wrap.

While you could use a blacklight or some other source of ultraviolet light, here in Arizona we have an abundance of sunshine and UV available, so outside it goes!

soaking up the sunshine

I left it out in the sun for about two hours, turning once to make sure I got all sides of the keys because the sun wasn’t exactly overhead.

Then I brought it inside, took off the plastic wrap and wiped off the developer with a paper towel.  Then I wiped them down with another damp paper towel before removing the craft foam mask.

after overhead

Ta-da!  The newly de-yellowed keys now match the Tippa’s ribbon cover almost exactly.  I did the “R”, “1/2 1/4”, and “BACK SP” keys earlier as a test before trying a whole row.

lever knobs after

The lever knobs look great too!  I took one from the other side for comparison.

By the way, you’ll notice I said “de-yellowing” instead of “whitening”.  That’s because this process removes the yellow cast caused by photodegradation, it doesn’t “bleach” or “whiten” it.

blue sears portable

This isn’t my typewriter, but it looks exactly like the Sears Portable I found at the local Goodwill.  I didn’t get a good “before” picture of mine, but it had the same two-tone aqua-and-blue look as this one, because the ribbon cover and body are blue plastic that has yellowed while the carriage is blue painted metal.


I did the ribbon cover first as a test, and it came out great!


Here’s what it looked like when I was done!  It didn’t get lighter, just less yellow.  Pretty cool, huh?