I'll tell you what works but nobody ever believes me. Armor-All, a detail brush, and being willing to put in the effort. It may take you an hour per flare. Spray it on, use the tips of the bristles to work the dirt and everything else loose, then wipe (scrub). Repeat. It works, my flares look like brand new and everyone asks how I did it. But as I said, nobody believes it.
"About Eastwood Plastic Resurfacer Matte Black Aerosol
With 7 year UV resistance, this is not just another black paint or wipe on coating! Eastwood Plastic Resurfacer actually reflows and resurfaces all faded and chalky exterior matte black plastic automotive parts and bedliner material leaving a factory fresh, deep black finish. It passed 3000 hours of ASTM D4587 QUV testing, that's equal to 7 years in the hot baking sun without fading. unlike many wipe on products It actually etches and reflows the surface embedding black resins into the substrate and maintaining the original texture. Compatible with all automotve plastics and urethanes."
1998 XJ Classic driver
1998 XJ Sport spare parts supply