mate its called oxidation,and paint pigment degradation,it happens to everything,even plastics degrade slowley due to ultra violet light breaking down paint pigment and plastic molecules in pvc/upvc,s and polymers.

getting these items re-coated will enhance and prolong the life of your plastics(bearing in mind the plastic oils that run through the plastic panels have probably been drying out since they where made) ,pluss the paint for plastics and polymer based items will eat into the first microns of surface area on your parts,giving back the flexibility and retaining some stregnth in these items,filling in the hair-line cracks at the same time, so getting them coated really is paramount,unless you can replace them with new (stored in a dark warehouse ) parts ,this is your real world option, if you have photo proof of before and after paint shots of the car,then most people would rather purchase a genuine ,freshly painted clean looking motor,than have to set aside some dosh to do it themselves,pluss you get to own a new looking motor again. and you could up your valuation for insurance,showing a perfect paint finish. just dont get "joey the gun" to finish your car for you!