While i do understand the aluminum 'chrome' coating process, I do not know how they prep the plastic parts.
The old coating is likely removed with an acid, Hydrochloric being an example.
The plastic part then likely gets some sort of primer or base layer to help smooth out surface defects. Don't know if it's sprayed, dipped or maybe spin coated. The surface must be perfectly smooth before 'chroming'.
The aluminum 'chrome' is applied in a vacuum chamber. The vacuum is required to remove contaminants from the atmosphere which would otherwise contaminate the coating. Parts are fixtured and rotated while the aluminum coating material is heated until it vaporizes..... a thermal evaporation process is likely used..... aluminum clips are hung onto a tungsten filament, current is passes thru the filament until the aluminum melts and vaporizes. The aluminum cloud will spread outward (by ilne of sight) and coat the rotating parts. Parts are rotated so all surfaces get coated without shadowing.
A clear protective layer would be applied over the aluminum. It could be applied via thermal evaporation, or it could be sprayed/dipped/spin coated afterwards.
At my previous job, we coated new plastic parts which only required cleaning before coating. The surfaces were still defect free. Aluminum or gold was used to produce the mirror coating (depends on which wavelength they need to reflect), the final protective overcoat was SiO2 (basically glass). End use could be parts for laser/bar code scanners, we even did some prototype headlight reflectors for Cadillac in the 1990's.
This process is also used to refinish mirrors in optical systems such as telescopes.
Wish I knew more about the prep they do before applying the aluminum chrome. That is likely a trade secret.
Years ago I would just wet sand the armrest base and paint it to match the door panel. Now I fear the sanding makes the part unusable for rechroming.
I made a career out of optical coating as a Thin Film Process Engineer, it's been a fascinating 30 years. Loved the R&D work but it gets heavy on the physics which can make my head spin!
