I would like to suggest a third option in the FX Panel-> Apply on-> Selection Menu.
It would be great, if you could choose to apply the effect on the key area. This way you would not always have to make the selection manually.
Here is a screenshot to make it more clear:
FX Panel Apply on Keys
FX Panel Apply on Keys
MAC OS 12.6.2 - TVpaint 11.7.1 PRO 64bit, Wacom Intuos Pro