How do you prevent a skin from forming on oil based paint while in the tin?

Answer 1

It seems to be AIR that makes the skin form, so your goal is to reduce the air-space at the top of the paint.  I keep an assortment of empty GLASS jars - and when I break open and use oil base paint out of a can, I then pour all the remaining paint into Glass jar(s) -- making sure each one is topped-up within 1/4" - 1/8" of the lid.  Put a double layer of (plastic) "Saran" Wrap - and s***w the lid on VERY TIGHTLY. When I use some out of the jars the next time, I find a smaller jar to hold the remaining paint. Eliminate the air -- that's the trick. You might be able to thin the paint sufficiently with paint thinner to top it up --  just be careful you don't add too much thinner!  

If you have paint that has already skinned over, what you are losing in that ugly golden goo is Linseed Oil. You can buy Linseed Oil in a paint store -- just add some to the paint to replenish what you've lost in the skin.  If you don't add the Linseed Oil, the finished paint job seems to be very "dry" and loses its luster.

Another hint I've read is to store the cans upside down -- but the threat of having a skin on the bottom of my paint scares me -- so I've not tried it.  

For short-term storage, you can float some paint thinner on top of the paint, but it's not a long-term solution.

Best wishes!