сам нашёл :
Brush mask most of what you're able to see, then use Visibility > HidePt > Grow. Keep clicking Grow until you have enough visible area to finish your mask. Make sure to clear your masks often, when you are not specifically using them. The Visibility features, as well as many others, will often leave traces of masks behind. You won't see them with your eyes, but they'll mess up your work if you don't make sure they're gone.