UV-glass photomultiplier tubes (PMTs) have poor photon detection efficiency for wavelengths below 300nm due to the opaqueness of the window material. Costly quartz PMTs could be used to enhance the efficiency below 300nm. A less expensive solution that dramatically improves this efficiency is the application of a thin film of a p-terphenyl (PT) wavelength shifter on UV-glass PMTs. This improvement was quantified for Photonis XP4500B PMTs for wavelengths between 200nm and 400nm. The gain factor ranges up to 5.4 ± 0.5 at a wavelength of 215nm, with a material load of 110±10μg/cm2 (894nm). The wavelength shifter was found to be fully transparent for wavelengths greater than 300nm. The resulting gain in detection efficiency, when used in a typical Cherenkov counter, was estimated to be of the order of 40\%. Consistent coating quality was assured by a rapid gain testing procedure using narrow-band UV LEDs. Based on these results, 200 Photonis XP4500B PMTs were treated with PT for the upgraded low-threshold Cherenkov counter (LTCC) to be used in the CEBAF Large Acceptance Spectrometer upgraded detector (CLAS12) at the Thomas Jefferson National Accelerator Facility.