Method #2: (only works with an extremely light weight backdrop) Acquire a photographer's backdrop stand (about $120).  Remove the top pipe and replace it with an 18' aluminum pipe. Its important that you use the aluminum because its lightweight for the strength and so won't sag.  Don't consider PVC--it will sag.  Now sandbag the upright stands.  Attach rings through grommets on top of backdrop and slide as before.  Warning: ONLY very lightweight backdrops will work with this method.  Don't even consider painted canvas.

 

'No', wire is not better than pipe.  If you use a wire it will sag over 18 feet, and also ripple if you put the wire through the grommets.

 

You can hide the backdrop behind scenery, or take a different approach.  If instead of painting or printing a backdrop, you could project on it.  Thus you can project different surface so that it can be part of any scene (a wall, or sky, etc) this way you 'hide' it in plain sight.

 

Also, (I work for a lightweight scrim manufacturer, so...) the obvious thing might be a painted/ printed scrim. That way you can work without removing the 'backdrop'.  A scrim allows you to make scenes behind the scrim appear and disappear. Change the light and you see through it.  Change the light again and you see the scrim, but not through the scrim.  Essentially you hide a scrim with light.

 

BTW--if you have trouble getting 18 aluminum pipe in your location, there is a solution to that, just ask.