basically UV has broken down the plastic. there is no real fix for it.
to give you an idea my original vermillion red panels were so bad that i could put my finger through the plastic just pushing on it. nothing was holding the plastic together.
because the plastic is so brittle you can't put any weight on it. if somebody got in and out of the car and put weight on the hand rest area they may fall through the panel and get hurt.
you can paint the plastic but any new paint will scratch off it very easily and it will continue to disintegrate.
The fiberglass re-enforcement won't last, eventually the fiberglass will De-laminate from the ABS plastic panel. it would be better to make a mold from the original and create a new quarter panel from fiberglass and finish with Krylon Wrinkle paint to simulate Camera Case black texture. I've seen people try and patch and re-enforce the panel and it will just keep falling apart.
they made reproduction panels and they are complete garbage stay away from them unless you have a track car and need a limited interior.
the only solution is to fabricate new panels from wood shape as needed and cover them with vinyl to make them look factory or cough up lots of money for decent originals. if you attempt to patch the originals with a filler just be aware that if somebody puts weight on it and the plastic is very brittle it will fall apart.
if your plastic is just falling apart on the surface and can take weight without breaking then you can easily Spray paint it with Krylon Fusion paint. it will eat into the plastic slightly and not require a primer.
alternative is using a Plastic Dye spray paint, however i found it does not bond as well as the krylon paint.
just for a reference this is what i had to deal with:
complete replacement was my only solution