  We report a 1.65 times increase of the PMT signal and a simple procedure of
application of a new wavelength shifting (WLS) paint for PMTs with
non-UV-transparent windows. Samples of four different WLS paints, made from
hydrocarbon polymers and organic fluors, were tested on a 5-inch PMT (ET
9390KB) using Cherenkov radiation produced in fused silica disks by $^{106}$Ru
electrons on a `table-top' setup. The best performing paint was employed on two
different types of 5-inch PMTs (ET 9390KB and XP4572B), installed in
atmospheric pressure CO$_2$ gas Cherenkov detectors, and tested using GeV
electrons.
