If your backdrop is more than 8 feet long - or if, like me, you need to splice on an extension - you're going to have a seam. My recent "backdropping" created a seam over 10' long(!) - but I (re)discovered a solution: Fiberglass Mesh Tape. It's the bomb!
Here's the extension - and you can see I have a looooooong seam.
And here is the mesh tape. It's sticky, so just stick it over the seam. It'll hold the topping together and won't let the seam crack. Be sure to sand the seam down all nice and even first.
Here's the tape down the entire length of the seam.
Then it's just a matter of adding topping, making sure to put on a thick enough coat to cover & hide the mesh.
And if you sand (and sand and sand some more) and add a couple coats of paint, it'll eventually come out like this.