Thanks for the suggestions - my suspect was contamination also.

I did have to put some petroleum grease on the air trap filter seal to stop it leaking - I'm going to replace the entire air trap as that is the only think I have done, although the K-36 sprayed fine.

I have been using a brand new Iwata Mini-jet, and the instructions specifically mentioned flushing it to get out all the preservation oil, which I thought was the culprit on my first attempt. I may switch back to my old Harbor Freight spray gun the next time to try to isolate the source.

Paul