Might be onto something here. I too have a separate hose I use just for painting. More importantly I have a good filter I connect when painting, here is my setup.

A belt-drive compressor (oil bath crank) will put a small amount of oil the the line. This is fine for normal air tool use but of course not for painting.

When painting I connect one of those coiled 3/8" yellow hoses to the compressor and run it up the wall then back down to my filter (pictured in the link above) to cool the air. Then I connect my "painting hose" to the filter and paint away, no additional filter at the gun. This setup has painted three planes with no fisheye.