<snip> I have a couple dozen opened bottles of Scalecoat and I routinely put thinned paint back in the bottle. They only gel when the cap is not airtight. Floquil bottles seem to have the best airtight seals.
and Aidrain Bridgeman wrote:
"Once a can of POR-15 is opened you have to use it all in the same session, but if you add a whiff of propane to the top of the can before you seal it it prolongs the shelf life so that the contents are still liquid several weeks later."
I am missing something because doesn't the old trick of storing the paint containers upside down still make an "air-tight" seal; although it cannot prevent air from entering the container upon opening.
Replacing the "air" with an inert gas such as nitrogen would probably work better than propane.
