>I'm confused the article didn't say anything
>about plastic and I thought the point was for it
>to stick on the wall?

Added to the "Murals on Canvas - How to" article:

Staple 1mil (thin) plastic onto the wall about 1 foot larger on all foue sides than the size of the mural to paint as a membrane between the wall and canvas.
Staple canvas over plastic and prime

if you do not put the plastic then the primer (gesso) will sip through the canvas and will stick to the wall so you will have a hard time tearing it off when done. plastic will just pill off the back of the canvas