Hi Song Dog
One other thing I could mention is that when we paint the edges, we lay the painting down on its back (face up) over the edge of a table or flat surface. Then we paint along the edge, moving the brush "upwards" from back edge to front edge. In other words, the brush goes along the edge of the painting, but does not first contact the painting anywhere near the front of the canvas, greatly reducing the risk of putting paint on the front.
This is hard to describe in text, but hopefully you get the sense of it.
Not sure what kind of tape is used to apply to the edge of the canvases, but it would be a specific art product, I'm sure, not the black tape that I used to apply to my hockey sticks, ha ha.