Hello Guy,
Maybe a bit late, but I want to join in anyway.
I experimented with different fillers, but what works best for me is Revell Plasto. It is rather "chalky" when you use it "pure" and it crumbles easily, but I now put a blob on an old bank card and add a few drops of Revell Contacta.
It now sand without crumbling, but it is not so tough that I sand off the surrounding styrene in stead of the fillings.
I have no experience with scribing it afterward, but maybe experimenting with the ratio Plasto/contacta will work.
Cheers,
Willem