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Preserving partially used paint and varnish
Posted By William Tindall
Date Wednesday, 1 June 2022, at 5:43 p.m.

Shown is a can of paint I partially used 30 years ago.  I used the remainder today.  I had a farm gate stolen at the time.  Bought a new one and painted it pink. Pink gate was never stolen. but I digress....

"Oil" paint and varnish both have similar chemistry, so what applies to one applies to the other.  They cure by reacting with oxygen.  Oxygen MUST be excluded or they will crosslink and become more or less solid and insoluble in anything, including the solvent in the varnish can.  So what to do to preserve a half filled can of remaining varnish?

The oxygen must be displaced from the air above the varnish in the partially filled can.  If the air is displaced with anything that dissolves in paint solvent this gas will dissolve, create a partial vacuum in the can and collapse it.  Therefore, propane, carbon dioxide and freon dusting products can not be used.  Bloxogen or compressed nitrogen can be used but they are an expensive solution.  A way to extend these oxygen free gases is to displace the oxygen with a gentle stream of freon dusting product.  These products are cheap and there is a lot of it in a can because in the can it is liquid.  Liquids expand many times their liquid volume when evaporated.  This stuff is much heavier than air so it is effective at displacing air in the can. Then displace some of the freon with a puff of Bloxogen or compressed nitrogen.  

The technique is to put the lid on the can and sneak the tube from the freon or nitrogen into the cracked can lid.  Gently add the displacement gas. Blasting it in will stir up the gas and suck in air.  

Never put unused varnish back in the can.  Once out of the can the varnish sucks up oxygen and if this varnish goes back in the can the whole can is primed for crosslinking.  

Store cans upside down.  If there is a tiny air leak it will seal with cured varnish.  If there is a big leak you will regret following this advice.  


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