I recently read about a product offered by Tetenal (only in Europe) that's used to preserve developer in partly filled containers - the product is an aerosol spray that prevents oxidation by creating a heavier than air layer of inert gas on top of the developer.(posted 8572 days ago)I was browsing through a Lee Valley Tools catalogue (a Canadian hardware and tool manufacturer and distributor) and came across what's called "Finish Preserve", which they advertise as preventing oxidation & drying of partly filled containers of varnish, lacquer, etc. Sounds like the same thing that is (inronically) made in the USA.
If you're interested, have a look for item 53Z21.01 at www.leevalley.com Current price is $12.95 CDN (about $8.75 US).