I realize this is an old issue, but I'm just now finding time to catch up and noticed this. 

The advice on drywall mud for scenery says that "it is also called Spackle." This isn't true, Spackle and drywall mud or Joint compound are different products with different consistency and behavior. 

Drywall mud is meant to fill in the joints when putting up drywall, the description in the how to response is correct for this product. Spackle on the other hand is generally for patching small holes and repair. It often contains a plasic component. It dries faster and is far less likely to crack. 

I find spackle to be the far more user friendly product for model railroading. Good for filling in around castings, making roads, touch ups and the like.

John