You can store opened paint cans for quite a while and not have them harden, skin over or go bad. It's easy.
When I open a quart can of paint to spray, I punch some holes around the rim of the can so paint can drip back into the can after pouring. After pouring the paint I need, I wipe that groove pretty clean, tap the lid back on securely and then
store the can upside down. That's the secret.
That way, no air can get into the can to allow the thinners in the paint to evaporate and dry out. It's that easy, I do it all the time and have done it for 15 years.
In fact, I just finished painting a ripper blade (rebuild thread soon) with John Deere semi-gloss "soft" black paint. This was from a partially used quart can that I opened 10 years ago. It went on perfect, dried perfect and no problems.
This has other uses and I've finally convinced my wife to store unused homemade spaghetti sauce upside town in jars to make it last a couple days longer. Guacamole in a plastic container won't turn brown if you turn upside down and slap the container on the counter to force guac around the inside edge. Things like that.
So, take your time, clean the rim and have your way with the remainder of paint in the can whenever you get around to it.
EDIT-photo is a ripper shank I painted two days ago with paint from a partially used can stored upside down around 10 years ago. Maybe 12.
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