The StopLossBag is interesting - an idea whose time has come, so to speak.
I just paid Sherwin-Williams $147 for two gallons of water-based paint. Good paint, but the price is getting outrageous. Of course people are going to start trying to conserve paint and varnish.
Shown below is what I use for water-based coatings. I will either attach a sponge to the lid, or put the can in a ziplock bag and put the sponge in the bag.
I have stored "used" paint this way for years and it works for me. I have 10 year old paint that is still useable - after stirring. Problem with attaching the sponge to the lid is you can't shake the can without messing up the sponge, but I just avoid shaking the can.
A wet sponge works because it maintains a "saturated" environment inside the can - so water cannot evaporate out of the paint, and the paint cannot polymerize (skin over) in the can. The paint can still separate in the can, but it cannot polymerize. Stir the paint and its OK.
I have not tried a sponge with oil/solvent-based paints or varnishes, but I do not see why it would not work for them also. Just need to use a sponge that does not react with the solvent...
I have also used a "sponge-in-bag" method. Put any lidded paint container in a ziplock bag along with a wet sponge. Keeps the contents "juicy" until next use. This "bag" method will not work for long-term, but it will keep an opened paint can juicy during a long job.
The sponge can be glued directly to a metal can lid underside, but for plastic lids , better to drill the lid and screw the sponge on. Put a metal -foil tape over the screw head (see pictures) to prevent possible air infiltration through the hole.
