I am spraying urethane primer . On the 2nd coat i am getting what looks like dust coming out of the gun in the primer.It is like the primer is dry and clumped as it comes out of the gun. It sands immediatly off but what is causing this?

Primer to thick or clogged vent hole in the cup. What kind of primer? Did you strain it?

It is urethane primer and no I did not strain it....

How long should i be able to keep the primer mixed in the can before spraying another coat?
The first coat went on great..nice and smooth... Second coat was rough. Hit it with 180 and the dust sized chunks come right off...

Depending on the air temp and the speed of the activator you used the potlife is about 45 to 90 minutes. Also a clogged air cap could be the problem. Take it off and slosh it around in a cup of thinner.