The kick & back corner panels on Big Blue were that way. I replaced the kick panels with ones with vents, but sanded the mealy yuk off the corner panels and got down to good plastic. Then I prep'd them and painted them with SEM and they look pretty good. You can see the sand marks if you look closely, but otherwise they are good.
I suspect the door panels were that way but someone had already sanded and painted them so I don't know for sure. But since they also put 6x9" speakers in them they had to go so I painted a new pair of panels from JBG.
Had I wanted to reuse old door panels like that I'd sand down to good plastic and then use SEM's texture paint first and then the color over that. The UV damage only goes a little way into the plastic, and the fact that the blue turns such ugly brown probably means the UV doesn't penetrate as far on it as other colors.
Gary, AKA "Gary fellow":
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Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI