To be strong, the second layer must be in intimate contact with the first layer. This is why orthopedic surgeons spend a lot of time rubbing down a cast. The plaster will usually set in about 5-8 minutes. When finished, check to be sure there are not any weak areas that give to finger pressure. Before moving on to rock castings, it is important to anticipate any other scenery or structures that will be part of or abut the mountain setting.
Any road needs to be well demarcated by now.
The road should be slightly wider than you really want it because the rock castings will take up some room. Track or future roadbed should be in place so you can judge clearance for rolling stock. Any tunnel through the mountain must be roughed in by now and the portals in place. Structures must have their footprint anticipated and preferably set in place with a piece of plywood. Rock castings require a rock mold.