I've tried several gases to fill the can, including homemade CO2 and propane. Neither worked, but if you want to make some co2, put some baking soda and vinegar into a large beverage container. The gas given off as it fizzes is CO2, and because it's heavier than air, it pours out of the container like a liquid....so you can fill your finish can up with it. You can test the level of CO2 in the container by lighting a match and sticking it in there...it goes out when it hits the Co2. Of course after that it's doesn't do crap to save the finish...at least IME. Propane is also heavier than air, so just squirting some into your can presumably removes the O2. That didn't work for me either. The next thing I tried was to vaccuum the can out. When those vac bags with the built in port were available I cut the ports out and taped them to the lid of finish can, drilling several holes under the port. I then sucked the air out with the little pumps you get for them..I actually crushed one can doing this. Sadly, that didn't work either. I tried putting marbles (and gravel that was rinsed off and dried) into the can to raise the level of the finish to the rim, and then put the lid. This actually worked really well in saving the finish, but it's a bit of pain separating the 2. So now I'm at what I'm currently doing. I use
Stop Loss bags. These work really well, though they are also a minor pain to use. But I've got 3 right now filled with 3 types of varnish, one of them is dated 10/21/18 and the finish in there is still good.
