The marble trick is OK, but still leaves a small bit of air in the can or bottle of finish.
I was going to buy a can of the inert gas product BloxO2. At the time it was over $17.00 per can. Then I realized I had a 220 cu.ft. cylinder of inert gas on the back of one MIG welder, and a 120 cu. ft. bottle on the other MIG welder. I use Ultramix 80-20 Argon/ CO2
Just open the valve and regulator, lift the wire feed wheels, put the nozzle of the gun in the open can... ready to quickly close the lid, and pull the trigger for a few seconds. Even if a bit of air is not purged out, it cannot contact the finish because the heavier than air gas displaces it.
Now I also have a 75 cu. ft. cylinder of straight Argon for my TIG welder. In a pinch, I have used propane from a Bernz-O-Matic torch, as propane is also heavier than air. A little extra caution is needed as propane is flammable. I could see someone like the Nutty Professor pulling the trigger on a self-igniting torch, and blowing himself up... again.
