- Old plastic changing colors or damaged
Plastics which have been exposed to the sun often fade because the ultraviolet light breaks down molecules in the surface layer.
Plastics can also age due to exposure to oxygen and other reactive gases in the air, which breaks down molecules in the plastic surface.
Both of these conditions can sometimes be seen if you have very old adhesive labels on a device that were never removed, and the plastic under the label is now a different (original) color compared to the rest of it.
Once molecular damage has been done, there is not really anything that will restore the damaged molecules to their original state.
Plastics often have a base color with additional coatings applied as paint, or chemically bonded-on plastics mating two types together into a composite.
The silver trim on plastics can be just a thin surface coating. The principle of buffing is to remove surface damage and reveal more material underneath. But there may be nothing underneath the silver trim except the plastic base color.
Bending or denting plastics often turns them white, and this whiteness condition is not reversible.
You may be able to remove fading with very light sanding to reveal undamaged plastic underneath, but if there is any textured surface finish you will remove it with the sanding. If the backside plastic is not the same color as the front, there is risk that the front side faded plastic layer is very thin, and sanding will reveal the other color.
You may have better luck just spray painting the plastic casing with colors that match the original, as this will preserve surface finish without removing material.
Though for consistent appearance you will need to paint the entire surface, requiring you to mask off areas that should not be painted.
Also the paint is unlikely to perfectly match the original color or appearance so you will need to paint all exposed surfaces on all plastic shell components so that their final appearance is the same.
- Old flexible plastic becoming brittle
Old flexible power cords have a tendency to become brittle and to crack as they age. There is really nothing that can be done to prevent this, and you will have to replace them.
Plastics such as vinyl or neoprene are very hard and brittle in their natural state. They become soft and flexible with the addition of plasticizers generally known as pthalates which are extremely thick and heavy oils that are mixed into the plastic during manufacturing. The plasticizers act like tiny straight coil springs embedded into the plastic and allow it to flex without breaking.
However as the years go by, these springy molecules can work their way out of the vinyl and pop out, evaporating into the air. This is the smell of new plastics that fades over time as it ages.
During manufacturing there are many plasticizer molecules that don't embed into the matrix, but just lay on the surface lightly wedged between other vinyl molecules. These wiggle out and evaporate more easily and are the cause of the strong odor of new plastics.
The more embedded plasticizer molecules stick around for years without an odor, but when they finally work their way out, the plastic becomes very rigid, and starts cracking when flexed.
There are products that claim they can restore old vinyl to "like new", but they generally only apply plasticizers to the surface. The chemical in the bottle may not exactly match what was originally used and may not be entirely compatible.
Exposure to pure plasticizers in a restorative can be bad for you, as they have been shown to harm biological processes through biomimicry. Some plasticizers are misinterpreted by organisms as if it were estrogen, which can cause feminization, development of breasts on males, and disruption of sexual processes in young animals when released into the environment.
‘Gender-bending’ chemicals found to ‘feminise’ boys - New Scientist, 27 May 2005
https://www.newscientist.com/article/dn7440-gender-bending-chemicals-found-to-feminise-boys/
A critical analysis of the biological impacts of plasticizers on wildlife
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2873012/
You are better off just replacing old plastic that has become brittle than trying to restore it. Once it has become brittle and cracks, it cannot be mended.
- Rubber that turns to sticky slime
Some types of flexible rubber have a tendency to age differently, turning from a solid into a slimy liquid. You will often see these as the rubber feet on devices, becoming sticky, and then degrading into a very thick and disgusting runny slime that gets all over everything.
These should be removed as soon as possible when they start to become sticky, and replaced with new rubber. If you cannot find an exact replacement, you can sometimes either just leave it off and don't have them if the bottom of the device won't scratch tables, or substitute something completely different, such as stick-on felt pads used for furniture.
