I have a model which I primered white brushing it on. Since it's white, i overflooded it a bit trying to get more coverage. Since I lost some details I wanted to wash it out and reprime. NO MATTER which fluid I sucked it in for days (spirits,chlorine, various home cleaners, brake fluid even maybe white spirit), the thing doesnt come off. And I rubbed it with brush while wet. Acetone probably would work but the model is plastic.... Well, I brush it on. I airbrush it on. Works equally well. It sticks better to the plastic then to the metal. There are two rules I follow. When airbrushing don't try to get coverage from the first two coats. Apply them really misty having a couple of minutes between them. The third one will cover everything and you'll see that you spent less primer then trying to cover from the first coat. I usually end up with 4 layers with the last being grey over black. Paint the model next night, or two, or three. That is it. But it still will chip from the metal when falling on a floor. And it doesnt like sanding when it comes to other plastics than the minis.
One more thing. The stuff in the dropper bottle smells and feels a bit different than in bigger ones. Like it has more resin maybe... The bigger bottles feel more close the the gesso. However according to Vallejo it's the same stuff just in different bottles.