900 hours and I am seeing some image retention. How to remove them?
In a few words, you can't.
Panasonic Plasma Screens have a movement option that slowly moves the screen around a few milimetres
I the burning is bad and all you can see is Sky TV's frames or Sport frames, the way to reduce the effect, and this is official, is to turn up the screen brightness on a white background. This will burn the rest of the screen to a similar level!
Unbelievable but having done that for a number of hours, when one increases the brightness to compensate for a burnt screen, it is slightly better.
(I do not advocate that, I would not let it burn in the first place.)
Hope that helps.