Marshalltown make a fibre tape gun. I want one. £50 I think but for the corners it would make it so easy to do. Then a little Easyfill or Troupet and it's sorted. This is a link to a 30 second demo video from another forum some guy has uploaded to his Google drive.
https://drive.google.com/file/d/0B0i5ujjihN2FUFh5ZVQzTVNldHM/view?usp=drivesdk
https://drive.google.com/file/d/0B0i5ujjihN2FUFh5ZVQzTVNldHM/view?usp=drivesdk