I’ll apologize in advance if this has been asked and answered.
Skin is formed on the surface of a can of varnish that has been left unused or unagitated for a period of time. I always skim it but certainly this skin is comprised of some solid component that was intended to be in there when manufactured and would affect the final finish. After I skim it, the varnish just doesn’t look quite right and seems to have a different flow consistency. So my questions are:
Is there a way to reconstitute this back into the varnish?
Add something to it?
Should I just trash the remaining can?
Thanks for your help.
Replies
A little skin is NP. For major projects I prefer to use fresh product. Really no noticeable difference though for most non critical applications.
The skin is not formed from solid matter in the varnish, but from polymerization of the oils in the varnish. In that process molecules of oils of plant origin bond together in the presence of oxygen to form much larger molecules, eventually becoming a solid. It cannot then be redissolved. Petroleum based oils do not polymerize. Laquer type finishes harden by evaporation of solvent, and can be redissolved.
Skin formation can be prevented by excluding oxygen--filling can with innert gas, filling with marbles to take up empty space (messy).
Tom
Sorry for delay responding to the valuable information. Thanks for the response. This all makes sense to me so I guess when the can is half full I'll pour the remaining contents into a smaller can or jar to keep the air out. Thanks again to both respondents.
Lee Valley sells a aerosol oxy displacer for paints or varnishers. works really well.
The best basic way to think of varnish is to realize that are no oils in varnish. Oil was used as an input to a chemical process in which the oil and a resin were chemically reacted to create the varnish. Once the varnish has been formed the oil no longer exists as oil. It is varnish that is polymerizing, not really one part or another. And, there is no way to disintangle the polymerized varnish to be usable again.
Edited 3/2/2007 1:28 pm ET by SteveSchoene
It would appear that you are defining "varnish" as the cured and hardened product, in which the oils have disappeared through polymerization, and I am considering it to be the liquid product, which surely contains oils. I don't know which is more proper. nor does it seem important as long as we both understand it.
Tom
Tom,
I believe what Steve is saying is that saying there is oil in varnish is like saying there are eggs in cake. Technically yes there are eggs in cake, as they are an ingredient, but once you mix everything together and bake it the eggs no longer exist as a separate entity. In other words, if you let eggs sit on the counter they will soon spoil, yet you can let a cake sit on a counter for quite a while and the eggs in it will not spoil because they have combined with the other ingredients and changed there chemical makeup. The same is true of the oil in varnish. It can no longer be separated from the varnish because it is now part of a new substance that we call varnish. It isn't oil, resin, dryers, etc. It is now just varnish.
So the film on top of a can of varnish isn't just oil, or just resins. It is cured varnish. If I am wrong about what you meant or anything I have said fell free to correct me Steve.
Rob
Rob,
I had not read the previous explanations to be consistent with what you have written.
I find your explanation the best way to explain the situation, and I'm glad someone found such a logical way of describing the process.
Rich
Rich,
I cannot take credit for that easily understandable explanation. I took it's basic form from Steve Mickley the host of the finishing forum at Wood magazine. He is very good at reducing technical concepts to easily understood ideas.
Rob
I think RobA did a good job of explaining it. What is in the can is a liquid varnish which is diluted to usability with solvent or thinner and a few additives and driers. (There are additives to reduce skinning over, for example. The varnish is a different chemical compound than any of the ingredients even were they just mixed together. The oil and resin are mixed and heated in a process that reacts them to become a new substance--varnish.
Sometimes varnishes are classified as long oil (like spar varnish) or short oil, (quick rubbing varnishes are shorter oil varnishes for example.) This has to do with the proportion of oil and varnish as ingredients, but it is not possible to create a long oil varnish by adding oil to a short oil varnish. Similarly, there is occasionally confusion over the rather large differences in how oil/varnish mixes perform compared to how varnish by itself performs.
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