Airbrush - streaky primer

Hiya guys,
Just starting my second model, uss enterprise into darkness model.
I've been having a problem priming the pieces, I'm using a badger 250 with canned air and the primer just ends up way too wet and runs which leaves not so nice looking marks.
This didn't happen with the first model, original series uss enterprise.
The only thing I've changed is putting a brass tube in place of the plastic feed tube. Could it be this is bringing up too much primer? It's the same diameter measuring by eye but in reality I can't be sure.
Or could it be using cans is the issue? Baring in mind I managed it no issues for the first model.
Any advice would be appreciated,
Thanks, Mark

There could be a number of things you need to check.

First, did you try going back to the plastic feed tube to see if the result change back to normal? Did you thin your primer too much? Can you control the air pressure? Do you have a moisture trap?

Hiya, thanks for the reply,
I unfortunately seem to have lost the original tube. May see if I can find a replacement as I do have a feeling it's because of it, perhaps I didn't notice it with the paint so much as the primer.
The primer is a 'no need to thin' bottle of stynylrez.
As a bottled propellant it's basically just on or off really. And no moisture trap.
I'll see about finding a new tube and see how that does.
Thanks again for the reply.

No problem.

If the kit parts have a greasy surface that can cause running, maybe clean a test piece and try again? Given the limited options you have, ensure that the thinner is well mixed,
try spraying from a bit further away from the model and move the airbrush more quickly so that less liquid hits the model at any one time and place.. If problems persist try a different primer, Vallejo have a good range of ready to spray primers, but I don't know if they are more viscous than the stynylrez.

You may be getting moisture in the line from the compressed air can as it cools.
I'm not going to try to explain it I'm just going to be lazy and paste it from a web site way smarter than me....

https://sciencing.com/canned-air-cold-5157676.html
"Canned air is actually not the same as the air we breathe. Canned air is commonly a mixture of nitrogen and other relatively harmless gases that are compressed to 40 to 70 psi (pounds per square inch) until they convert to fluid form. Some of the gases are actually heavier than air, and when they are sprayed out, they will fall to the floor. Some of these gases can also displace the oxygen in the air, so it is always best to use them with proper ventilation.
Because these gases are compressed to liquid form, when the can is turned upside down and sprayed, liquid will actually come out of the nozzle before it has a chance to convert to a gas. The reason this doesn't occur when the can is right side up is that there is a pocket of gas at the top of the can.
The reason the can gets cold after being used is due to a process known as adiabatic cooling. Adiabatic cooling is a property of thermodynamics that states that when a gas is placed under a high amount of pressure, a significant drop in temperature will occur when that pressure is released. The high level of compression necessary to convert a gas into a liquid allows a massive amount of gas to be stored in a relatively small space, and when that gas is released to a large space, it rapidly expands to fill the space.
This results in a drop in its internal energy, and it absorbs a large amount of heat from the surrounding air. This heat absorption results in the cooling effect. This is one of the reasons that every can is labeled with a warning stating that it should never be sprayed onto skin due to the fact that this rapid absorption of heat can easily cause frostbite. The frost that forms on the can is a result of the condensation of water vapor in the surrounding air."

One old trick to try is placing the can in a tub of luke warm water as your using it. That's supposed to keep the pressure even.
You don't get an even pressure from them as you use them. I've heard they start out at around 45 psi and go down from there.
I usually run my air brush at 15 psi from a compressor.

Recommend if possible getting an air compressor with a tank, two gauges and a moisture trap. I have a cheap Campbell & Hausefeld recip compie with 2 gallon tank that I installed a trap on using a standard pipe thread section of brass pipe. Pretty quite compared to the original Badger comp I had, kind of a loud hum. I spray at ~35-40 psi setting with minimal thinning, just have to keep cleaning the tip to remove the "spike" of paint that builds up (bit of thinner and a quick spray into the rag)

The output from cans is too variable to make it worth the trouble, easier to just use a spray bomb...
Regards, Robert