Shake the primer well. Spray it through a large straw into a container of some sort (I use plastic shot glasses). Then use a pipette to transfer it to your airbrush cup. I don’t even let it gas out any more because it wouldn’t need it if sprayed directly from the can onto the model. It DOES need to gas out however if you are going to store it in a secondary closed container.
I like this method, as the un-decanted paint is still stored safely in the best possible container….the original can. Once there is not enough pressure to spray any more paint, I might puncture the can to drain the remaining drops.