Shotcrete can be placed as thin as 1 to 2 in. (25 to 51 mm), but will do little more than to provide a new surface to apply the plaster. To do a proper job you need to remove all loose and deteriorated existing shotcrete and should likely add in a layer of reinforcement or use structural fibers (either steel or synthetic) in the shotcrete mixture. The surface preparation should be done to the standards outlined by the International Concrete Repair Institute (ICRI).