An autonomous spray application system for a structural Thin Spray-on Liner (TSL) is targeting productivity and safety on underground coal continuous miners by replacing steel mesh, and has broader applications as surface support and confinement in metalliferous mines and civil tunnelling. Polymer chemistry, coating formulation, geotechnical assessment and the autonomous application system are being developed within an integrated package as part of a continuous miner automation project. The polymer-based TSL system, dubbed "ToughSkin", has significant benefits over conventional steel mesh, including; more automatable by spray application, complete skin confinement, active skin reinforcement as a fast-curing composite with substrate, and effective fire retardant for the polymer and underlying coal. This paper investigates distance sensing systems that will enable an autonomous manipulator to adapt nominal spray path standoff distance and application angles in order to spray consistent thickness (nominally 5mm) over an irregular, unknown topography. Two alternate transducers are assessed: firstly, a 2-Dimensional LIDAR mounted on a linear axis is used to produce a 3-Dimensional surface profile that may be used to generate an adapted spray path; and secondly, a robot end-effector-mounted ultrasonic distance transducer is assessed for simple wall-following techniques to dynamically adapt a nominal spray path. Each transducer is tested for range, accuracy and repeatability for a variety of surface colours, angles and finishes, including a coal piece.