Ouch, sorry for you m8
I'm no expert, but in my past experience with scratches I would of thought that as it is deep enough to see the undercoat/primer then it's past polishing out :thumbdown:
Like RCZ1 says if your nail "picks" at it then it's more difficult to completely remove. And being on a darker car it will probably also show up more.
You could try one of those coloured scratch removers, there a few on the market now.
Apparently the Mantis one is very good, but I'm not sure if it comes as coloured ?
Only other option if you cant live with it, is one of those smart repairs, but if they paint it they will have to blend in with the whole rear quarter, unless they have another scratch removal / polishing technique :eusa-think:
Anyway m8 I hope you get it sorted. :thumbup: