# Prevent Paint from Skiming Over



## CJ Courtney (Dec 26, 2007)

Guys,every time I use up about 1/2gal of paint in a gal container of oil base paint it skims over.I always seal the can well by pressing in the middle of the lid while installing the lid back on the can.It don't happen while I still have about 3/4 of paint in the can.Just when I get 1/2gal or less.I don't leave the lid off for any length of time.Just long enought to pour some paint out.Any ideals on how I can save the rest of my paint?Thanks CJ


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## vermontpainter (Dec 24, 2007)

CJ Courtney said:


> Guys,every time I use up about 1/2gal of paint in a gal container of oil base paint it skims over.I always seal the can well by pressing in the middle of the lid while installing the lid back on the can.It don't happen while I still have about 3/4 of paint in the can.Just when I get 1/2gal or less.I don't leave the lid off for any length of time.Just long enought to pour some paint out.Any ideals on how I can save the rest of my paint?Thanks CJ


Peel that skin out with a stir stick, then stir and strain whats in the can prior to use.


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## The paint whisperer (Oct 18, 2007)

I can't help you with this noe CJ. 
And I would love to know the answer to this one myself.


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## ProWallGuy (Apr 7, 2007)

Seal the can, and store it upside down. The skin will then be on the bottom.


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## The paint whisperer (Oct 18, 2007)

Thanks, proWall.
I just went down my basement and turned all of my oil base paints upside down. 

I normally did like Vermont said.


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## vermontpainter (Dec 24, 2007)

This topic makes me think of a tip I was given a few years back by a 68 year old floor finisher. Urethanes are one of the worst offenders on the skimming over. The skimming is caused by the air in the partial can causing drying on the top. This old timer told me that he would take his last partial can at the end of a job and add clean marbles to it to reduce the volume of air inside the can. Sort of like adding ice cubes to your drink raises the level of the liquid in the glass. I had never heard that one before. PWG's idea is more practical.


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## dragula (Nov 19, 2008)

Saran wrap.

Or introduce CO2 somehow...Someone here said a breath would help...But an exhale still contains around 16% O2, as the body only absorbs ~5% during inhalation.


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## NEPS.US (Feb 6, 2008)

A splash of thinner on top before I seal usually helps.


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## ProWallGuy (Apr 7, 2007)

Along the lines of Scott's response, an old-timer once told me when he would cap off a can, he would blow into it and close it quickly. This forced out the oxygen, and replaced it with carbon dioxide. He claimed the paint wouldn't dry/skim over like that. Maybe one of our resident chemists can verify or debunk this theory.


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## Workaholic (Apr 17, 2007)

vermontpainter said:


> Peel that skin out with a stir stick, then stir and strain whats in the can prior to use.


Ditto
I never really found it to be that big of as deal to lose a skim layer of paint from a can sitting for an extensive period of time.


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## nEighter (Nov 14, 2008)

lets think about this. Wine drinkers use a decanter and put NO2? into the void to save the wine from oxygen. Could this same method not be used? Displace the O2 with NO2? Or is it they displace the O2 with CO2? Hell I can't remember.. and ex's dad was into wine and that was my intro to the wine culture.


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## johnpaint (Sep 20, 2008)

NEPS.US said:


> A splash of thinner on top before I seal usually helps.



This has always worked for me.This also work for water base just have to use water instead.


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## TooledUp (May 17, 2008)

There's no sure-fire way but once you put the lid on give it a good shake. It doesn't always work but I've had some success.


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## NACE (May 16, 2008)

NO2 is the trick. Exhaling into the can delays the skinning. Oil paint dries by solvent evaporation and oxidation. If you remove the oxygen, the skin will not form. Try wippets.


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## Tonyg (Dec 9, 2007)

But the BIG question is does the light stay on when you put the lid on??






Have a customer that puts wax paper in all his remnant cans to keep it from filming. The only practical answer is to trickle a small amount of solvent on top.


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## Bender (Aug 10, 2008)

Doesn't the skin keep it from drying out?


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## timhag (Sep 30, 2007)

johnpaint said:


> This also work for water base just have to use water instead.


You get a NO DUH! for this one. :yes:


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## TooledUp (May 17, 2008)

Bender said:


> Doesn't the skin keep it from drying out?


The skin gets thicker until it reaches the bottom of the can lol.


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## timhag (Sep 30, 2007)

TooledUp said:


> The skin gets thicker until it reaches the bottom of the can lol.


:yes::whistling2:


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## wppainting (Mar 2, 2008)

Bloxygen is designed specifically to keep oils from skimming.

From the website: "Bloxygen uses a special blend of inert gases to drive the oxygen out of your container. Simply blow the oxygen out of your container with Bloxygen and then seal the lid. The heavy, inert Bloxygen gases sink down to block oxygen from the liquid surface. Because Bloxygen is heavier than air, it will separate the liquid surface from any air that may remain in the container."


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## dragula (Nov 19, 2008)

ProWallGuy said:


> Along the lines of Scott's response, an old-timer once told me when he would cap off a can, he would blow into it and close it quickly. This forced out the oxygen, and replaced it with carbon dioxide. He claimed the paint wouldn't dry/skim over like that. Maybe one of our resident chemists can verify or debunk this theory.



Debunked. An exhale still contains ~16% O2.... Otherwise CPR would be worthless.


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## RetiredChemE (Mar 30, 2020)

*It’s not oxygen that’s the problem - it’s evaporation*

I also have that annoying problem of skin formation on the top layer of oil based paint, which thickens with time. The issue is evaporation of the paint solvent from the top layer because of the vapor space is at a partial pressure (vapor concentration) below its equilibrium vapor pressure. This would happen despite any relatively inert gas also present in any combination, - nitrogen, oxygen, CO2. So, here’s the mechanism and the solution. The problem: The solvent evaporates and a skin forms at the solvent-deprived liquid surface. Additional solvent evaporates and the skin thickens, until there’s enough solvent vapor in the overhead air space to then have sufficient partial pressure concentration there to eliminate the driving force for evaporation. The bigger the air space, the more the evaporation and the greater the skin thickness. Adding paint thinner to the surface could help by adding a solvent vapor to the air space for minimizing surface solvent evaporation. Putting a complete layer of plastic wrap could help by sealing the liquid surface and impairing the evaporation rate. Adding golf balls, marbles or anything to the can to minimize the volume of the air gap would also help minimize the amount of skin. Remember: It’s not air or oxygen that’s the problem. The problem is surface solvent evaporating to the air space above.


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## CApainter (Jun 29, 2007)

RetiredChemE said:


> I also have that annoying problem of skin formation on the top layer of oil based paint, which thickens with time. The issue is evaporation of the paint solvent from the top layer because of the vapor space is at a partial pressure (vapor concentration) below its equilibrium vapor pressure. This would happen despite any relatively inert gas also present in any combination, - nitrogen, oxygen, CO2. So, here’s the mechanism and the solution. The problem: The solvent evaporates and a skin forms at the solvent-deprived liquid surface. Additional solvent evaporates and the skin thickens, until there’s enough solvent vapor in the overhead air space to then have sufficient partial pressure concentration there to eliminate the driving force for evaporation. The bigger the air space, the more the evaporation and the greater the skin thickness. Adding paint thinner to the surface could help by adding a solvent vapor to the air space for minimizing surface solvent evaporation. Putting a complete layer of plastic wrap could help by sealing the liquid surface and impairing the evaporation rate. Adding golf balls, marbles or anything to the can to minimize the volume of the air gap would also help minimize the amount of skin. Remember: It’s not air or oxygen that’s the problem. The problem is surface solvent evaporating to the air space above.


Loss of paint vehicle by evaporation in a sealed container is unlikely the sole contributor to skinning. Particularly, when talking about oil based paints that chemically cure through oxidation. 

Heat will cause any solvent (or vehicle) to change to a vapor state. The molecules in this agitated state expand, rise and are carried away. However, in a well sealed container, they will not necessarily be carried away. Especially when the vapor condenses due to cooler temperatures and the vaporized solvent returns to a liquid state.


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## RH (Sep 7, 2010)

Eleven year old thread. No wonder there's a skin on the paint.


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## Zoomer (Dec 17, 2011)

RH said:


> Eleven year old thread. No wonder there's a skin on the paint.


👍


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## Lightningboy65 (Mar 12, 2018)

NEPS.US said:


> A splash of thinner on top before I seal usually helps.


old thread, but it could have just stopped here. about a quarter inch (more than a splash) on top works on fuller bodied paints, anyhow. When time to use, most of thinner will still be on top....pour off what remains on top ,mix paint , strain if needed, good to go.

Turning can upside down is useless and makes the problem worse. Skin will still form, and it can not be removed in tact.


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## kmp (Jan 30, 2011)

I solved the problem many years ago. I stopped using oil base paint.


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