Chemical treatment vs. surface layer removal( the following is the edited body of a PM sent to a member discussing chemical treatment using an oxidizer versus physical/chemical sanding for the removal of age related plastic discoloration )As for using an oxidizer for chemical treatment of discolored plastic, the salon care developer is the right stuff. If has a long shelf light and being a creme mixture, it stays on the plastic rather, does not run off, as a watery mixture would. You only need to use the amount necessary to cover the plastic. Volume developers are not an immersion process. Vol 20 may be a bit weak. You can read the link at the post below for information on creme developers and decide for yourself which you want to use. You can buy it in a smaller quart bottle at your local Sally Beauty Supply store. It's inexpensive, a little goes a long way and it has a long shelf life. I bought Vol 20 and 40 so I could use them as is or mix them together to get 30.
http://www.classicrotaryphones.com/forum/index.php?topic=17038.msg175932#msg175932That topic started with "what polish to use" and turned into a chemical treatment topic at this link in the topic. You may want to read the topic from this point on to learn more about chemical treatment of discolored plastic.
http://www.classicrotaryphones.com/forum/index.php?topic=17038.msg175836#msg175836=====
Now, as for this type of restoration work, there is a lot of information posted on the forum from 2009 to about 2018. Chemical treatment means the use of a strong oxidizing chemical, bleach or peroxide (salon hair care stuff or the retrobrite formula), to get rid of the discoloration.
Chemical treatment is a process that causes a chemical reaction on the plastic's surface, a reaction is hoped will reverse the chemical reaction that took place over many years due to inherent bonding degradation, atmospheric conditions, light and cleaners/polishes coming into contact with the plastic. UV light discoloration is easy to spot in that one side of the phone, the phone facing the sun light, will have received more UV light over the years and discolored more that other areas on the phone.
Chemical treatment using an oxidizing agent is not an exact science and without getting into the reaction that changes pi-level orbital bonding in this PM, the result from the use of chemicals varies from the chemical used to the plastic it is used on. Every housing came from a different batch of plastic and how that batch was mixed to come up with surface pi-bonding that created a desired reflective color is unknown and different from batch to batch. Over time, the pi-bonds are broken by any or all processes mentioned above and the color changes, the plastic becomes discolored. What color the original plastic changes to, the discoloration, depends on how the pi-bonds are broken and/or re-arranged and the length of the time over which the bonding degradation took place.
From experience reported in forum topics, some colors are improved by using an oxidation reaction using bleaching or peroxide and some are made worse. It seems lighter original colors are better "restored" with chemicals than darker colors which are made worse. Blue, red and green, from what I remember being posted, are not good candidates for chemical treatment. White, ivory, beige and pink can be improved to some extent. Not sure about yellow but probably a good candidate.
I don't want to get into the physical chemistry of reflective color generation in this PM other than to say that discoloration is the original plastic color being changed over time as its surface pi-bonds are changed. Changes to what color and cast is a function of the external agent (including UV light) hitting the surface, the length of time, the strength of the external agent and the plastic mixture originally poured/cast.
Bleach and peroxide are merely external agents of another type that produce a chemical reaction that only affects or changes in some way the very thin layer of the plastic's surface. After the treatment, that thin surface layer becomes free to once again change, to discolor, due to chemical changes in the "restored" plastic by way of inherent bonding degradation (the stability of a color change due to use of an oxidizing agent is suspect), atmospheric agents and conditions, light and cleaners/polishes.
I tried to restore a pink housing (shown below) some time ago using peroxide.

After treatment (left image) it looked good. I put the phone in a box and 6 months later discovered it was back to the salmon color that the peroxide "removed" during chemical treatment. The chemical change in the surface color by treating it with peroxide reverted back to the darker, salmon color in a short period of time, and in a dark box.
Over time, I've seen moss green change to a darker green, pink change to a salmon color, beige and ivory getting darker and blue change to a greenish yellow tint. AE made an orchid colored phone which was notorious for changing to blue over time (discolored image below). It was thought that the dyes and/or pigments themselves used to create the orchid color were unstable an more easily degraded over time. You can see the original orchid color inside the housing and handset in the second image below. This phone would have to be physically sanded to get back to the original color. There is no amount or type of chemical treatment that would improve this phone, make it look like it did right out of its mold.


Below is an example of an orchid AE phone in the process of discoloration. Chemical treatment probably would not work on this housing either in that the purple changing to blue chromophores are uneven in concentration across the surface (discolor variation) and chemical treatment reacts evenly on the entire surface at the same time.

The only real (time consuming and tedious) way to get any phone back to it's original color is to physically remove the discolored surface layer by paper and/or chemical sanding.
As for a step by step procedure for chemical treatment method to remove discoloration, it's nothing more than cleaning the plastic surface to make sure no oils or waxes are present which would keep the bleach/peroxide from getting to all of the plastic evenly. Oils and waxes will prevent the oxidizing agent from reaching the surface. When using the peroxide creme oxidizer, a phone housing will fit into a 2 gallon Ziploc bag (freezer bags are better in that the plastic bag is thicker and less prone to tear). The salon volume developer is added to the bag, which ever volume is thought to be needed. Volume 30 is a good starting point. Simply close the bag and smoosh the creme all over the outside of the housing. Set the bag in a warm area under either sun light or in a UV light box. Every 10 minutes, re-smoosh the creme on the body to prevent splotches due to an uneven chemical reaction. Peroxide needs warmth and UV radiation in sunlight to activate the chemical reaction (a warm spot is needed along with UV light in sunlight to activate the peroxide molecules that come into contact with the plastic - a foaming may appear which is the breakdown of peroxide into water and oxygen with oxygen being the chemical reaction that affects the discolored plastic. If using sunlight, it is important to turn the housing every 10 minutes (when re-smooshing) to provide even amount of light exposure to the entire housing during the treatment period. It is also important to pay attention to housing areas, such as cradle ears, which always seem to be in the light and as such can be over exposed and lead to over lightening of the ears.
After a period of time, the color change can be seen through the bag and when the desired color is reached, simply take the housing or plastic part out of the bag and rinse it off. Dry the housing and check the color. If not happy with it, put it back into the bag, add more of the creme and repeat the exposure/smooshing cycle. You may have to add or change the peroxide in that it does wear out, it breaks down, and no longer produces oxygen. In a warm area with bright sun light, this process can achieve the desired color change in a few hours or less.
Bleach is similar to peroxide creme in that it also oxidized the plastic surface. Household chlorine bleach also produces oxygen in a similar but different chemical reaction. Bleach water seems to work with or without sunlight. It seems the best environment is warm bleach water and sun light to speed up the reaction between the released oxygen and the plastic's surface. Bleaching will react in the dark as long as the temperature of the bath is a warm room temperate, say 80 to 90 degrees F. Liquid bleach is diluted with warm water in a tub or pail large enough for a phone housing to be submerged. Since the bleach bath is bleach mixed with water, there is not need to smoosh it around but stirring the bath every 10 minutes is recommended to prevent "hot spots" from appearing on the plastic. If the water mixture (distilled water and bleach) is stirred, the concentration of bleach will always be the same on all parts of the plastic. Distilled water is recommended in that no one knows what is in tap water and what affect it may have on the bleach reaction on plastic. Depending on the plastic itself, the amount of discoloration, the temperature of the bath, the use of UV light and the concentration of bleach in the bath, bleaching could take anywhere from an hour to many hours. The housing should be removed from the bleach bath periodically to check it progress. When satisfied with the color change, remove and rinse with water.
With peroxide or bleach, it's a trial and error process. I suggest searching for and reading the good topics posted on the forum by members who have successfully used chemical processing to adjust discolored plastic and to note those topics reporting that a color was ruined using chemicals. Ruined plastic usually means a splotchy surface or undesired color was created by oxidizing the plastic's surface, and in those cases, all is not lost. One can always sand off the outer "ruined" layer to get back to the original plastic color. Discolored plastic is a very thin surface layer and the original plastic in the original color can be restored by sanding off the outer layer, discolored layer.
It has also been noted that the chemical changes in the the plastic which cause discoloration also changes the physical properties properties of the plastic. I have many times found that the discolored surface layer to be a very hard plastic resistant to both chemical and paper sanding. While the original ABS plastic dissolves quickly in pure acetone, a discolored surface disolves very slowly with pure acetone on a cotton cloth and sanding required a smaller grit to cut through this hardened plastic. As soon as that discolored layer is removed, the underlying original plastic sands well, as expected, with higher grit paper or chemicals (pure acetone on original color ABS is very aggressive and pure acetone on discolored ABS plastic has little affect).
I had a severely damage soft plastic ivory 302 housing (and the physical effect of a handset cup shielding the housing from discoloration can be seen in the red circle in the image below).

The surface on two sides turned a crispy, toasted brown. I broke off a few pieces of that housing and put them in pure acetone. The inside of the chips dissolved away in a few hours leaving the toasted brown surface intact, undissolved. After more than 2 years in that acetone bath, the toasted brown surface has still not dissolved.

That is proof positive that the chemical change that occurs on the surface to produce discoloration also hardens the plastic, makes it something other than ABS. The polymer become glass-like with the increased hardness resistant to solvents, which would normally dissolve the plastic quickly, and sanding.
Plasticizers, their role is plastic hardnessThere is another type of chemical, in general called plasticizers, that is added to polymerized plastics to give the plastic certain physical properties. This would include making the plastic softer, more flexible than the pure plastic polymer and increase the flow and thermoplasticity of a polymer by decreasing the plastics viscosity when in melted form (easier to inject into a mold). There are chemical groups of plasticizers and the decision to use which one with some plastic to achieve a certain result is complex.
Plasticizers are usually inert organic materials with high boiling points and low vapor pressures. Esters are commonly used due to their favorable physical interactions with high-molecular-weight polymers. These are the chemicals that interact with the polymer to increase its flexibility. One plastic supplier said "The mechanical properties of plastics differ with plasticizer levels. Lower plasticizer content yields a harder surface, higher heat resistance, greater rigidity, higher tensile strength, and better dimensional stability. Higher plasticizer content increases impact strength." It could be the case that an external agent, such as UV light, which causes discoloration can also degrade the plasticizers and in doing so make the surface plastic harder and less flexible. That may be why the underlying, original plastic beneath the hardened, discolored surface layer not only retains its original color but also retains its original "softness" (lower resistance to solvents) and flexibility, or less brittle and we all know that old plastic becomes more brittle and breaks more easily.
Over time, there are several factors which can lead to migration of plasticizer out of plastics surface such as temperature change, humidity change, mechanical stress, and weathering. This is seen in old telephone cords becoming stiff and brittle to the point of cracking. Since phone plastic that is not discolored is also not hardened, it seems that something is happening along with discoloration that also breaks down or removes the plasticizers from the surface of the plastic making it harder, or solvent and sanding resistant compared to non-discolored plastic on the same housing (the inside, for example).
A common example of plastic with and with out a plasticizer is rigid and non-rigid PVC. Unplasticized PVC (or rigid PVC) is used in applications such as pipes, siding, and window profiles while plasticized PVC (or flexible PVC) finds applications in automotive interior trim, cables, PVC films, flooring, roofing and wall coverings. It seems that phone plastic in the process of being discolored also suffers a loss of its plasticizers, at the surface, making color restoration (regardless of method) more difficult.
Chemical treatment, as the means to remove discoloration, is a trial and error method when first starting to use either bleach or peroxide (including retro-brite). It is possible to test the bleach or peroxide on the inside of the housing to see what they will do to the color, to the plastic. While the inside will not be discolored, this test can determine if either oxidizing agent will harm the plastic. And even after doing that test, there is no guarantee of what will happen when using that agent on the housing since the exterior plastic will already have been chemically changed (it's been discolored) so using the tested chemical agent on the exterior may not react the same was as it did on an interior spot.
The above is being sent to you in a PM because, while I want to give you what I know, I want your topic to hopefully generate replies from others, not be ended by a long reply from me. I'm not sure how many members are still around that have done restoration of discolored plastics using chemical treatment but most of their pioneering work has been posted on the forum. Getting someone to give you a step by step will be difficult in that each person developed their own procedure, each may be slightly different and many won't take the time to share their work if not already posted.
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A few weeks ago, I paper sanded a moss green 500 housing that was painted grey. I used paper because of the paint, the scratches in the housing and the hardened dark green surface plastic that was resistant to acetone.
Below are the before, in process and after images. This image shows the 5 layers of color that were on this housing. In the center image and above the dial hole from left to right, you can see fiber strapping tape residue that looks light green due to is capturing sanding dust. To the right of that is the gray paint. To the right of the gray paint you can see a yellow plastic which is plastic that was color changed, discolored, by the solvents in the paint. Next comes the hardened darker green typical of WE moss green phones and to the right of that, the original ABS moss green plastic (which looks very light with its 320 grit surface texture). The image to the right is the completely sanded housing which was sanded up from 320 grit surface to 2,000 grit and then final polishing with Novus 2. The sanding process removed all discoloration, all scratches,all dings and revealed the original moss green color (chemical treatment does not remove scratches and dings).

The darkened, discolored moss green layer was so hard in some places (due to more exposure to UV sunlight) that I ended up using 80 grit wet sandpaper to cut through it. That took off the paint, the hardened dark green surface and the scratches (180 and then 320 grit paper was used to get the housing ready for 400 grit paper). Once the hardened plastic was removed, sanding the original, softer green ABS plastic up to 2000 took a very short time. The entire housing was sanded up from 320 to 2000 in about 3 hours.
The reason the 320 grit surface looks so light is a function of random light reflection. Once polished smooth, the reflected light is quite ordered, uniform, parallel while shows, to the eye, only the light from each part of the surface, not scattered light from all over the surface due to rough sanding marks which makes it look lighter. All of these images were taken under the same light source with the same camera.
The image below shows a badly discolored blue phone (handset and bezel show green) that I restored by physically sanding off the handset and bezel. That made the original ABS blue plastic, just beneath the discolored surface layer, the new surface color. The discolored plastics have been perfectly restored to the original color and match the housing well.

The housing didn't change color because it had been professionally painted by a WE refurb site using Polane. The paint was the exact color, a perfect match to the color that showed on the inside of the housing. A very nice match. Polane is a very hard, 2 part resin paint that does not change color over time and in this case it nicely shows how green a blue plastic phone can become over time.
( Clicking on the images below will enlarge them showing more detail than the thumbnails above )