I've used modelling clay to form the continents on the Planet Mongo. Slartibartfast was right: it is fun to do the fiddly bits on the fjords!
I returned the globe to its stand because I was afraid I had made the mountains too lumpy for the globe to turn. It was true, the volcanoes of Tropica got stuck on the arm of the stand.
A word on the clay I used. I picked Das, which is non-toxic air-cured modelling clay. Trivia: I had classes in University with the son of the inventor of Das clay. The family was very wealthy from the sales of Das - anybody in the Arts knows that the real money is in selling art supplies - so their son was able to take advantage of the best instruction. I recall him as being very conscientious and generous, a genuine nice guy.
I'm used to working with Super Sculpey - see my homemade Neconomicon on JSVB Post #1294. However, Sculpey has some drawbacks: it needs heat to cure and turpentine to thin. The heat would melt the globe which would be okay if I was modelling the Klingon moon Praxis. Turpentine is a toxic irritant. Das, on the other hand, is not toxic and it uses water for thinning.
I was worried that the continents would not stick to the globe. Das on its own is a little tacky, but it isn't sticky. Slathering the globe with water and wetting the pieces of Das cases it to thin a little, and when Das is thin it's very sticky. Wet Das also binds strongly to whatever it dries to, mostly my fingers and modelling tools. So, Das is messier than Sculpey, but I simply cleaned my fingers and tools with a wet rag.
Das has a peculiar sticky sliminess when thinned and worked, so it's not exactly like clay. After it dries for an hour or so, it becomes malleable and very clay-like. If Das dries too quickly, it will crack, so I kept the globe in a container with a moist paper towel to make sure the dry-cure went as slowly and evenly as possible. Das does shrink as it dries, but my continents were small and thin enough that the shrinkage was barely noticeable. Proper curing takes around 48 hours.
I should have taken pictures of the sculpting session. My hands were too gunked up to work the camera, though. Das goes on light grey and dries to ivory white.
So, back to the mountains. Since they were too tall, I had to sand them down. Cured Das takes to sandpaper very well, so it wasn't difficult to abrade the mountains and coastlines down to a manageable level. The picture above is before the sanding, so the finished globe looks quite a bit less lumpy.