I'm working through "Basic Blockout", and I spent a lot of time working on the bottom fins. When I turned to the tail fins, I was unable to apply a mask. I use the Mask Brush with size 45 px and strength 1, + (not -) set, and mask does NOT work. In the same fiIe, I tried to create a new object, (cube, control -5 to add a mesh) and again the mask does NOT work. I quit Blender and reopen fiIe and mask still does NOT work. I clear mask and still mask does NOT work. I go back to a previous version of my shark when I had not started the bottom fins and I WAS able to apply a mask. There must be some setting that I inadvertently set in my shark file?
Control M did the trick. I poked around and found overlays. So I learned 2 more things...thank you so much chrislim and spikeyxxx for your help.