Ok, so I am going to make a mold and later spray some clear coat and I know nothing about spray guns, so here we go 
1- I ordered duratech primer earlier, In application guide it states that primer can be sprayed with 1.5-2.5 tip and 35-50 psi. I had bought the following cheap spray gun at harbor freight " http://www.harborfreight.com/20-oz-high-volume-low-pressure-gravity-feed-spray-gun-47016.html " Required pressure is 20-50 psi so it should work fine, right? what should I use to clean the gun? Should I thin the primer?
2- what is the difference between operating and working pressure for a gun, when I was looking for spray guns online I came a cross for example: Operating pressure at 40 PSI, working pressure at 10 PSI. ???
3- I want to try sherwin Williams hpc15, can I use the cheap harbor freight gun for that too?
Clean the clear using thinner and clean the duratec with acetone or mek for best results. I recommend not breaking down those throw away guns tho. Pour in half a cup shake and spray. Turn air full blast as well as feed and turn the fan pattern nob open and closed. Then shoot some clean chemical and soak the cap in a cup of mek.
I spray clear with the gun full blast. Be sure to also buy a water separator and a bubble filter for the gun. So you don’t shoot water on the part or mold.
The cheap guns work just fine.
Hpc can be shot heavy so … Don’t get scared . It gels fast and cures right away