I used 'Loctite Professional filler - plastic padding'. You can get a tub of it from Halfords for around £6. Though i've found the hardener only lasts about half of the putty. It might feel quite rubbery once it's set but it's perfectly fine to sand down (i'd reccomend an electric sander like a detail sander or orbital, and don't apply too much, just enough).
I think there's still some tiny little bits on the front of the panel that need to be filled but it's 99% done.
I think there's still some tiny little bits on the front of the panel that need to be filled but it's 99% done.