It’s widely believed among finishers, especially refinishers, that once you add a fish-eye eliminator to a finish and spray it, your spray gun is forever contaminated. You can’t ever get the fish-eye eliminator, which is silicone, completely removed, and little bits of it will continue to break lose and cause fish-eye problems.
This is not correct and shows a misunderstanding of what silicone actually is. It’s simply oil with a very low surface tension. Silicone is noticeably slicker than mineral oil, for example. Rub a drop of whatever brand of fish-eye eliminator you use between your thumb and finger and you’ll feel the difference.
So getting silicone, which you might have added to a finish to eliminate fish eye, out of a spray gun is simply a matter of washing the gun well. Usually, you can do this adequately just by sloshing some clean lacquer thinner around in the cup a few times and spraying it out.
The same principle works with a separate pressure pot and hose, though it will require more thinner.
You could also take the cup and gun apart, of course, and clean each part with thinner.