Hey,
I agree with AndyS remove the stuff youve got at the moment and start a fresh (the humbrol stuff isn't great for this application, a friend also tried it
)
I brush my liquid mask (I also use RCS liquid mask) I start with the usual washing the body etc and then I apply a thinnish coat using a soft bristled brush (I find a 1in brush is about right), Dry with Hair dryer (on hot/warm) setting until it is dry (takes a min or so) then repeat for approx 4 layers.
I then leave the last coat overnight before cutting and painting. Done right this leaves a preety smooth and even coat that removes with ease.
Personally I wouldnt spray liquid mask on with the same brush I was airbrushing with (IMO the brush is much too small, I think a cheap air gun maybe better for this?)
Best of Luck