Rich's suggestion is simple and effective, but I would (as usual) suggest using masks for almost everything. I am a mask zealot.
With masks you can dilate, erode, blur, mean curvature blur (to fix the squareness of dilating and eroding), offset, and more!
It is a good idea to make a selection of the initial mask white area and save it as a channel.
With masks you can dilate, erode, blur, mean curvature blur (to fix the squareness of dilating and eroding), offset, and more!
It is a good idea to make a selection of the initial mask white area and save it as a channel.