I've used different patches before, and I've seen terrain tutorials where people use spackling, specifically DryDex Spackling. It goes on pink and drys white, which is fantastic. Spackling is light, thick, and spongy. When it dries, it has a tendency to crack on terrain surfaces. I used some for about half of my fantasy house walls and had to recoat all of it with mud so I wouldn't be afraid of people touching it. Spackling is great for filling in cracks or small details like that, but I don't recommend it for large projects. I think it dries inside the sealed container faster than mud does as well.
I'm super dried out!
I'm not saying spackling is bad, as there's a place for it. I just don't think that place is terrain projects.
For the rest of the fantasy house, I used Sheetrock Ready-Mixed All-Purpose Joint Compound, which has turned out to be pretty sturdy. It doesn't dry as fast as spackle does, but the additional strength that it adds to the piece is worth it. This also means that it'll stay fresh longer.
You make me so wet.
I haven't done too much with the mud yet, but since I don't have a foam knife I decided to cut out some stairs in my next terrain piece. While cutting, I made a mess out of the foam. Because the stair area was so damaged, I decided to cover it with mud and just sculpt the brick onto the foam.
The area is still wet, so I don't know how it'll turn out. It was much faster than carving into foam is, and there's already a natural texture to the blocks. When it's all dry, I'll compare it to carved foam.
So what I really wanted to say is, I know a lot of you (who make terrain) use spackle. There's a similar material comparably priced that works a lot better (as far as I can tell).