I got a Schneider pola filter on the 10-20 which must be very thin due to the wide-angle.
Not only it's very difficult to remove (I have to try and get my nails into the sawtooth at the side) but what's worse, due to the extreme angle the pola shows a gradual effect which is bad in skies.
So I'm very reluctant to use it anymore. Lesson learnt, and price paid.

Thanks for the trick. It's extreme, but if it works...

There are no Borg. All resistance is fertile.