Tried to spray some epxoy primer on my car using the recommended mix (4 parts primer to 1 part activator) and at the recommended pressure (40 psi). What I got was a coarse dry coat even after adjusting the gun. What have I done wrong?

Slow down the gun movement and lay more on or perhaps your tip is too fine. You have to match the tip flow number to the spec on the primer for it to spray at normal speed. If you dont know the number check with your paint rep for some literature on the product. With epoxy primer or most 2k primers for that matter it dries so quick that you really got to lay it on heavy. Better to try and hit it hard and perhaps get a run to see how far you can go than to lay it on too dry and have to sand it all forever which will probabaly take most of it ff anyway. Good luck man.

Absoutly. If have not ever sanded epoxy primer, then you know to be carefull. That stuff, once dry id rock hard.

Are you sure It's 4:1 on EPOXY primer...?
Or, are you talking 4:1 ratio for 2k CATALYZED primer.
(which is more common)
I don't know any EPOXY that's 4:1...(but I'm not familiar with all brands...quite a few, but not all )

You are correct. It is CATALYZED primer. Actual primer is Nason Selectprime 421-17.

That makes more sense...
What's the temperature like in Texas... That's probably another part of your problem...You CAN reduce the primer, for extreme temperature, even though not necessary.
Check the tip size in your gun, as mentioned above, ideally, it should be 1.6-1.8, for primer. Is it a new gun or an old one laying around that has not been cleaned properly?

I apply it in thin coats, let it flash between them.

The primer has a long shelf life, the catylist does not. I didn't read the label where it said to use the catylist within 30 days of opening.

Anyway I had some 421-17 left over this summer from painting my wife's car in June of '03. I sprayed two doors, and it looked dry until I sanded- it only dried on the surface and was gooey mush below. I had to use lacquer thinner to wash it off, as it would not harden, I called the store, and they said the activator (catylist) once opened needed to be used within 30 days.....just like it said on the can I didn't read.