surface primer

hello got a question.

i use vallejo surface primer 74.601 grey.

but when i use it for my airbrush i get little drops all over it.
the airbrush is clean and not block with paint.
i think is it possibkler its because i dont have enough pressure of air
or because i need to thin it down eventhough its for an airbrush paint?

It is absolutely normal. Just apply more primer to get a well covered surface. Don't thin down primer. I use 25PSI, if you have such pressure it is ok.