Thought as much. I guess it's like anything you clean, it will get dirty again.It is ultra violet damage, any oil or other wipe on treatment will be limited. On some parts you can abrade back to good material if you can accept a sanded finish. But the likes of a bobbley scuttle, this is not practical. It is possible to blow it over with a flame but again only temporary, although longer lasting, it takes real skill to get it even and you run a risk of damaging the part or adjacent parts. The only practical solution then is to spray paint with an appropriate paint.
Hi I have found this to be pretty effective and cheap as well! One bottle lasts for ages and really easy to apply with a rag after washing and drying the car