I like Gypsolite as my surfacing material. Its natural color is light gray, and it has a very gritty texture. I squirt a bit of dark brown craft paint into a batch of the stuff to give it a tan color. This picture shows a section of this light tan Gypsolite after it's set, and another section, to the right of the engine, after I've applied a green wash, turf and flocking. Both areas were applied directly over pink foam.
I bought a big jar (I think it was 8 or 10 pounds) of this stuff from my LHS. If you want, you can look around and find it at building supply places in huge sacks for a better price. I only use it as a skim coat, so after 3 years of construction I've just started my second jar. For me, it's convenient to have a material that I can mix up a small batch of, simply by adding water and any coloring I desire, and have it ready in a few minutes. When you're mixing up a multi-ingredient concoction, it's a lot more effort just to get started. I prefer to do my scenery in small sections anyway, so this almost-ready-to-go approach works better for me.
It takes an iron man to play with a toy iron horse.
Goop is supposed to be an alternate type of scenery shell than the plaster products that have been used before goop. I wouldn't substitute any of the two basic ingredients which are latex paint and celluclay. The addition of plaster or plaster containing material (such as sculptamold) is going to change it's consistency and set up - working time. The alternate is to just go back to plaster over a scenery form (plaster soaked paper towels or plaster cloth), then paint it and apply the ground foam materials.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
Possibly...both have cellulose in them. Personally, I would resort to a gypsum-based product, like Plaster of Paris. It can be had for a few dollars at the craft section at Wal Mart. Sculptamold would be more costly, I would think, for the work you need out of it.
Could I use sculptamold in place of the celluclay?
I was watching one of the MR DVDs in which "goop" is made. It is four parts: celluclay, vermiculite, white glue, paint in equal parts. Add 1 capful Lysol to keep microorganisms from growing. After building a foam cake as a mountain ridge base and painting it quickly, some areas still look a bit too layered. I noticed this also at a recent train show. To help fill in to imititate the smooth, rolling hills of SE Ohio & W.V., I want to try this recommended ground goop.
Jim