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If I were to design a gravel road, I would probably start with wall plaster with coloring in it for the dirt. Before the plaster dried, I would use a fine ballast on it and press it into the plaster so slight ruts would develop.
On my layouts I've used Real Dirt by I believe Hi Ball. Or, I have sand from Sedona, Arizona that I used for a county road. I apply it just like any other ground foam, first diluted white glue, apply the material and then wetting agent followed by Scenic Cement.
Fine Cinders from Woodland Scenics works well also.
Greg
Here's a gravel road made from the sand from Sedona. -Greg
Start by putting down the sub road using plaster or spakling compound. Remember to put in the tire ruts. for soem details, take a toy car and roll it over some of the places, leaving tire ruts in the plaster. cover with fine ballast or even cinders. when its set, you can go back with a little clear epoxy and fill in some of those tire ruts to depict water in the ruts!