When we finish a work of painting, it is convenient to conserve the spare painting for later retouches.
Step 1: Logically, We Close the Can Firmly.
But if we close the can with the whole air that has inside, the oxygen will combine with the painting and it will create a hard layer on the surface.
Step 2: How Do We Eliminate the Oxygen?
It is very easy: 1) we add inside the can some drops of an inflammable liquid that it is compatible with the type of painting: alcohol for the water-base paintings, gasoline or solvent for the oil-base paintings.
In the upper left corner of the photo you can see a dropper with lighter fuel. Four or five drops are sufficient. More than that can alter the painting chemically.
Step 3: Warning: Adults Only!
2) immediately light a fire to the drops of flammable liquid.
Step 4: Put the Lid
Contrary to what one could suppose, when putting the lid it doesn't take place an explosion, but instead, considerable vaccum is created inside the can, which strongly sucks the lid.
The fire fades quickly because the oxygen wastes away, and then the painting will last very much more time without losing it's quality significantly.
If there is little painting and the can is of big size (1 liter or more), it is probable that the can is deformed due to the interior vacuum. But that doesn't affect in anything to the painting, and there is not any possibility that the can can produce spill of painting due to that contraction.