I have been unable to diagnose this.
I am spraying Endurovar Flat with my Fuji Semi-Pro 2-stage HVLP.
It is not spraying easily. The air is flowing. The tip is not clogged. My air is 100% open, and the fluid dial is about 80% open.
The first time I tried, it took about 30 seconds to begin flowing. But the material flow was very low. It strengthened as I kept spraying. The flow gradually increased until I was satisfied with the delivery.
I let the gun sit for 2 hours between coats. The second application took about 1 minute for the gun to 'warm up' to reasonable flow. On the 3rd coat, it would not flow - even after 2 minutes of depressing the trigger.
I am using the default 1.3mm air cap. General Finishes recommends a 1-1.3mm air cap.
I have sprayed gallons of Enduro Clear Poly with no issue. Between coats, I often have to pick the scab off the nozzle, but it flows fine after that.
I am not using the basket strainer. I found with Clear Poly it restricts the uptake too much.
When the Endurovar flowed during the 1st 2 coats, it laid down very well; there was no spitting or sputtering. The coat was even and as expected.
I emptied the gun and sprayed soapy water through the system (as I have always done for the Clear Poly) and it immediately sprayed strong and full.
I don't think the issue is clogging or a bad seal on the cup; the vigorous flow from the clean water shows this not to be the case.
I don't think the issue is too small an air cap because of the GF recommendation and my positive experience with Clear Poly.
The only thing I can think is that maybe the FLAT sheen finish is more viscous. I've not sprayed FLAT Clear Poly before (only SemiGloss and Satin).
If I get no other rec's, I'll probably try thinning with water. I hate doing that.
Any help appreciated.