An airless spray gun is an expensive investment and needs special care for storage, to avoid freezing & damage from moisture, which can cause rust. When I put my airless away for any period of time, I pump it full of a mixture of oil (e.g., 3 qts. of [any] non-hardening oil), paint thinner (e.g., 1pt), laquer thinner (e.g., 1/2 cup) and alcohol (e.g., 1 cup, pref. denatured). The thinners help clean the system of build-up of any paint, the alcohol mixes with both water and oil and, of course, the oil lubricates. This seems to work better than just pumping the system full of thinner or oil.
– Kelly Craig