Why does oil paint skin over in the can?

I find even with new paint whether a small or large tin, after i've used a third or half or whatever %26amp; even thou I hammer the lid on tight, the paint skins on the surface next time i open it. Why is this?

Why does oil paint skin over in the can?
You aren't stirring it enough. Usually, if you just use it as is, you take out the titanium oxide that floats to the top, on your brush. This is a key ingredient and may cause it to skin up after a period of time. It happens to oil based paint on dry days. Is there a serious reason to be using oil based paint? You should only use the oil based primer on certain things that require it... paneling....metal (even though behr has a latex primer that sticks to metal).





You can use latex paint on top of oil based primer... and you shouldn't use oil based primer unless necessary.
Reply:Maybe its from the air hitting it, even after the lid is put on there is still air in there to make it happen. So that they top of the paint starts to dry.
Reply:I hope you don't think I'm being a smart butt but the real answer is.... its just dries. when you stir up the paint, it gets on the sides and dries. It does this for latex paint as well. If I were you I would only use latex unless whatever it is your painting specifically ask for oil based paint. They messed up paint when they took the lead out long ago but hey...for health reasons they had too. They now use Titanium so when you shop for paint, look for how much titanium or percentage of titanium is used in the paint, the more that is in there the better the coverage will be.
Reply:It is because there is air between the paint and the lid that dries it out. If you store your paint cans upside down then the skin is at the bottom.
Reply:Thanks GreekQT. I like that.