Any gas other than oxygen will work.
The problem is that there aren't too many compressed gasses you can buy
in small quantities.
What I use is butane:
Just buy a short piece of brass or aluminum tubing from any hobby shop,
or use a piece of larger plastic tubing (from expanding foam caulk,
say), and slip it over the dispenser nozzle of the butane cannister.
Now, when you're ready to seal your can, put the lid on the can but
don't pound it down yet.
Lift one side of the lid and slip your butane dispensing tube under the
lid. Pull the butane dispensing tube backward toward the butane
canister, and butane will flow through the tube into the can.
Butane is heavier than air, and will gravity segregate to the bottom of
the empty space in the can, forming a physical barrier between the
liquid and any air remaining in the can, and that's just as good as not
having any air in the can.
But, what's important and that most people don't realize is the oil
based coatings (including alkyd based polyurethanes) absorb oxygen from
the air all the time the lid is off the can. So, if you start painting
at 2:00 PM, and seal up the can at 4:00 PM, it's a waste of time to put
butane in the can when sealing it up. That's because the oil based
coating was absorbing oxygen for two hours, so there's plenty of oxygen
absorbed into the surface of the oil based coating to form a film while
What you should do instead is:
A) remove any dried film on the surface of your can of oil based
coating, and stir it to ensure that the solids are uniformly dispersed
in the liquid.
B) pour off as much oil based coating as you think you'll need (or a
little more) into an old soup can, disposable cup, or whatever.
C) immediately inject butane into the can of remaining oil based coating
as described above, pull out the butane dispensing tube and let the lid
sit on top of the can while the butane inside warms up and expands (so
you don't have any serious pressure inside the can).
D) Now pound the lid down on the can to seal it.
E) Now go and paint from the soup can or disposable cup. Don't pour any
unused paint back into the can if it's been exposed to air for over an
hour. Better to store that in a smaller container (like a glass or
plastic jar) so that it doesn't absorb significantly more air, and let
the dried film form in that smaller container. (you can buy pint and
half pint miniature paint cans at any place in your city listed under
"Containers"). Better to put a thicker coat of poly on for better
protection of the substrate than to have to throw more dried poly film
into the garbage.
REMEMBER: The butane is a liquid inside the butane cannister. If you
hold that butane cannister at an upside down angle when injecting
butane, you'll be injecting liquid butane into the can. If you then
push the lid down onto the can so that the resulting liquid butane
transforms into a gas inside the sealed can, it'll build up
considerable pressure. Sometimes the lid will fly off shortly after
sealing the can, other times it'll fly off when you next go to open the
can. So, after injecting the butane, it's best to just let the lid sit
loosely on top of the can to allow any excess butane to leak out of the
can before pounding the lid down. Butane gas is heavier than air, so as
the butane liquid vapourizes into butane gas, it'll push the residual
air out of the can.
Hope this helps.