Polymers like acrylonitrile butadiene styrene (ABS) are adversely effected by UV exposure, both visually (by yellowing) and with regard to other material properties . What's worse is that the degradation of the material continues for a long time after the initial UV exposure (see  for quantitative observations of this effect in other polymers). Can these effects be stopped after the fact?
Context: The unsightly degradation of ABS is a problem for collectors of old computer hardware. "Retrobrighting" techniques  (using hydrogen peroxide and UV/heat) can de-yellow the old plastic very effectively (albeit with unknown side-effects on the material) but the improvement has turned out to be temporary. The re-yellowing occurs in the same pattern as the original yellowing from photodegradation and it does so even if the material is protected from further UV exposure! 
Discussion: There are many dubious claims about the causes of ABS yellowing and the effect of "retrobrighting". Most often the yellowing is claimed to be caused by bromine and the hydrogen peroxide is claimed to "bond with bromine free radicals and restore the color" [eg. 3, 5]. I did not find any evidence to support these claims so far.
While radicals play a role in the photodegradation, I think the yellowing could rather be due to chromophores like polyenes being built up . The hydrogen peroxide might convert the carbon-carbon double bonds of the polyenes to epoxy links, thereby destroying the chromophores  and restoring the original color. (Intense exposure to direct sunlight also seems to destroy the chromophores temporarily(?) ). The problem is that this does not remove the causes of the post-exposure yellowing and therefore over time the yellowing reoccurs . (If the hydrogen peroxide would truly capture the radicals as often claimed, I'd assume that the yellowing would stop permanently.)
Question: Is there any known and practical method to arrest the degradation of polymers after UV exposure? In particular, if this continuing degradation is caused by the presence of radicals in the material, can these radicals be neutralized or removed somehow?
Update: I'm reading about hindered amine light stabilizers (HALS) which are thought to scavenge the radicals involved in photodegradation. I wonder if HALS have any effect if applied to already aged and exposed plastic. Does anyone know?
Yousif E, Haddad R. Photodegradation and photostabilization of polymers, especially polystyrene: review. Springerplus. 2013;2:398. Published 2013 Aug 23. https://doi.org/10.1186/2193-1801-2-398
James E. Picket, Reversible post-exposure yellowing of weathered polymers, Polymer Degradation and Stability, Volume 85, Issue 1, July 2004, Pages 681-687, https://doi.org/10.1016/j.polymdegradstab.2004.03.008, PDF (linked under YouTube video ): https://drive.google.com/file/d/1vE41ASE14kZVGjEFz_tsiLRLK3c928Eh/view
"HOW TO CLEAN & WHITEN YELLOWED PLASTICS" http://www.retrofixes.com/2013/10/how-to-clean-whiten-yellowed-plastics.html
"Retr0Bright (or RetroBright) treated plastics re-yellowing even with minimal light exposure?" https://www.classic-computers.org.nz/blog/2013-01-15-retr0bright-only-temporary.htm
"What makes ABS plastic turn yellow?" https://chemistry.stackexchange.com/a/74620
Perifractic's Retro Recipes, "Incredible Retrobrighting Discovery! Deyellow Plastic with just Sun?! No bleach/dismantling!", May 4, 2019, YouTube, https://youtu.be/8P1OVj0IcqY