I once read about a guy who used to hold his breath and blow into the paint can before sealing it - the idea being that he's introducing CO2 which settles to the bottom keeping oxygen off the varnish. He claims it works. ??

What you stated about the varnish might be true but to what extent. Theoretically, the second you open the can you have O2 infiltration. Oil based paints skin over on top and in some ways it protects the rest of the paint from the oxygen. Acrylics don't do that - they thicken as a total unit. If you thin older thickened varnish it will still work but the molecular structure is slightly different than a new can. There's no sense of making people feel panic about a previously opened can of paint. It is kayak building and not NASA. I've used many old half cans of varnish and have thinned them and had no problems with adhesion, peeling, gloss etc. Actually with oil base paints, both things occur. Evaporation of solvents and oxygen combining with molecules.