Activity

It depends what brand/type of primer you have. Some don’t need to be thinned.
If you got primer by Tamiya (just a wild guess) you need to thin it with cellulose thinner. Any kind will do. You can get the expensive one by Mr. Hobby (the bottle with the blue label), the even more expensive one by Mr Hobby which contains a retarder (blue label with yellow writing) or cheap generic one you can get at any hardware store or online.
Thin down until you get the consistency of milk, at first outside the airbrush to get the mixture right. Ones you get the hang of it mixing in the airbrush is possible, just put the thinner in first, because the fluid direct in the nozzle mixes not quit as easy when doing a back flush, especially when the fluid, in this case paint, is rather thick.
I hope this answers your question. Should you need any more information, please get back to us.
Good luck and happy building!