Scenery for your slot track!
Or....foam, foam on the range...........
____I have done several model railroad layouts using these methods and some slot layouts also. The advantages over hardshell plaster are numerous and the cost isn't any greater if you scrounge the foam. It's easy to apply over small areas at a time, cleans up quickly, doesn't make a huge mess while you're doing it and it's just wonderful to smack a speeding car into a foam rubber cliff rather than a hard plaster one!
But hey, I'm weird....maybe a plaster cliff is more thrilling to smash one of your beautiful creations into?
The first photo shows the track on bare board. Foam track supports are in place and it's all ready to be boxed in.
The next photo shows the filler pieces hot glued around the track. These are made of a good quality corrugated cardboard. Yes, it does hold up well. Remember, this is only a filler, the track is supported on high density foam blocks.
Next, bits of foam rubber are hot glued to the cardboard. Use your imagination. The object of this particular spot on the raceway is to model an excavated dirt and sandstone cliffside that has grown over a bit.
Okay, the foam is in place. Now it is coated using latex caulking. Find a color and shade that suits the terrain you are modeling. I chose a light brown that does well as dirt and sandstone peeking through. Just squirt it out of the gun and spread around with your fingers or using a wet brush. Work it into the foam for a porous look or spread it thick for a solid look. Experiment, there are a lot of effects that can be done here.
Once the latex caulking has dried overnight, it's time to add the base coat of grass or turf. This is done exactly as you would on hardshell plaster. Sprinkle the ground foam on and then wet it down with a sprayer....finally apply the fixative and let dry. I use a 1:1 mixture of Elmer's White Glue and water, with a touch of alcohol to aid in wicking. Note: you might wish to add some drybrushed highlights to the latex if you are modeling rocks. Using gray latex and adding a wash of shades of gray, sienna and such, then drybrushing with white will add to the effect. Just use the same basic techniques found in model railroad books for this.
Here are two shots of another section. The first with all the foam and latex applied and the structures in place temporarily to mark positions.
Next is the same section with the grass base and gravel added. Please, use gravel only in areas that will be protected.....it can make a mess of a deslotted car! This area will have fencing, bushes and structures installed to prevent this.