If you want translucent, consider Chameleon Scrim...won't curl, won't fray, can razor cut the edges and can paint without sizing.  Use dilute acrylic paint.  You can save much time and cost by choosing the color to be the lightest color in your composition.  So lets say that you are painting  these drops to make a framed procenium, framed with leaves and vines.  You could buy the green color, and paint the highlights and shadows of the leaves/vines--presto, very little painting.    Say you needed about 7 feet wide and 20' tall on each side and 40' wide and 7 feet tall across the top.  Finished scrim ready to paint would cost...about...$600 --already has grommets and a pipe pocket on the side pieces.

 

I'm not sure what you mean about what fabric would give a rock look.  You can use several painting techniques to make rocks.  You can also drape fabric over chicken wire to make rocks, so would you explain?

 

To answer Joe's question, I think he is talking acrylic when you say plastic--not oil-based.

 

He's right that you don't HAVE to size, but if you don't the absorbant fabrics (like muslin or canvas) will use a lot of paint.  So you can spend money on size or paint.  I'd do it this way.  If I was just spraying dilute paint on, I wouldn't size.  If I was painting with full strength paint, I'd size to save money on paint.

 

For rock stairs, build plywood stairs, adding a few irregular upright shapes of plywood (boulders).  Cover with strips of carpet or with chickenwire and then carpet--stapled into place.  While covering add some irregular lumps of wadded cardboard or paper (little rock mounds), and also  put scraps of carpet or carpet padding over those upright plywood shapes (filling out the boulders).  Don't forget to add a few irregularly cut strips of carpet to the steps to make them look rough hewn. 

 

Now paint the whole thing with some scrap grey paint.  Assign some kids, they will slap paint on it.  You can even assign a few kids to throw sawdust at it, while the other kids paint. You can even mix up the odds and ends of old wasted paint.  Anything grey, brown, greenish will look rock-like.  Note: don't mix acrylic with oil based.

 

Give kids brushes and sponges or rags and have them hit the surface with some texture from a darker color paint, by sponging lightly or dry brushing, or rolling the rags across the surface.

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Now have adults or trusted kids add highlights, shadows and moss to the rocks.

 

I've done this before and it turns out great.  BTW if you use carpet, if its a high pile, turn it backwards and use the back--unless you light furry rocks!  If you can't get carpet, carpet padding can be used and covered with canvas.  You use carpet because it actually makes a pretty secure surface, if an actor accidentally walks on a boulder instead of a step, they won't rip through it.