Nah there are a few vintage computers that share this awful varnish that makes a sticky goo when you try to clean it with IPA, the PB100 is one of them. You've got two solutions ; either you don't ever touch it with IPA, or you wipe it entirely by pouring a large quantity of IPA on top of the board, brush it until it's all gone and doesn't stick anymore... (but easier said than done)
The brightness : I've repaired three PB100 inverter boards and none of them seem to work properly after recap. I believe it's the potentiometer itself that's not working well, some active component, or maybe the tube itself (which by the way consumes a LOT of power, more than half the power a PB100 draws is the LCD backlight)