[StingRay]

Polypropylene is unbelievably tough stuff. Same stuff modern front bumpers are made of. it's actually tough to stick any kind of paint to. Pressure washing, paint strippers and the like will wok fine. Pressure washing might get a lot of that oxidized stuff off too. Blasting works great and when painting again it'll have a little tooth into the plastic. Use low pressure and a small blaster for the job. Not trying to kill the panel with an industrial pressure blaster. An adhesion promoter should be used before paint and possibly a flame treatment of the panel as well