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How long to keep paint

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How long should I keep old paint that has been opened and used. I find that even if l close the jar quickly after some time the paint begins to become difficult to work with, it clumps or doesn't cover well. This goes for both bottle and spray paint. Any ideas???

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Unfortunately, the answer is "it depends".

I'm still using a jar of Pactra "Military Colors" interior green that's at least 35 years old. It's one of the big round jars with the mustard colored top. I'm sure there's not an atom of the original solvent left in it any more but as it thickens up, I just add a little solvent and it's good as new.

 

If it's a solvent based paint, as long as it's not completly hardened you can usually add solvent (I ususally use laquer thinner) and stir it up.

 

If it's water based, you're probably out of luck if it's hardened. Acrylics polymerize as they cure, not just dry like enamels and laquers. Once the molecules of acrylic link up, you can't thin it back down.

 

I have seen Model Master enamels separate into a layer of pigment and a brown layer of what I guess is the binder and have the whole thing kind of gel. I just toss them at that point.

 

I think a key to extending paint life is to clean the threads of the jar or the rim of the tin of paint before you close the container. Some folks store them upside down using the theory that it will keep air out.

 

 

I've never heard of a can of spray paint going bad as long as it has a good valve and it has propelent.

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Agreed! I have Revell paints that are at least 10 years old and have opened them occassionally and they are fine! These are all enamels.

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Acrylics polymerize as they cure, not just dry like enamels and laquers. Once the molecules of acrylic link up, you can't thin it back down.

 

My understanding is that "polymerizing" or curing (a chemical reaction after the solvent evaporates that eventually turns the whole paint film into a single and hopefully tough molecule) is what defines an "enamel" type paint. What modelers call "acrylics" are technically "water-based enamels", and what modelers call "enamels" are "oil based enamels". "Acrylic" just identifies the material that forms the paint film. Note that across the pond the term "acrylic" sometimes refers to "acrylic lacquer" (automotive touch up paint) - which can lead to all sorts of misunderstandings on the web...

 

I've heard, and seen it happen enough to believe, that adding thinner to a bottle of enamel can cause a mostly full bottle of paint to cure rapidly (a few days) even with the lid tightly sealed. I haven't been scientific enough to know if its all enamels and all types of thinners, but I try to avoid doing that on general principle (I mix the thinner and paint in a separate container).

 

Don

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With acrylics, it all depends on the container and your sealing habits. I have some citadel acrylics from the 90s that are still perfect, (in pots like modern P3 acrylics) then I have some Citadel acrylics from a year ago that are dried out (in their stupid new pots). - Think it's either marketing genius or buyer stupidity, not sure ;)

Edited by dsteingass

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Acrylics polymerize as they cure, not just dry like enamels and laquers. Once the molecules of acrylic link up, you can't thin it back down.

 

My understanding is that "polymerizing" or curing (a chemical reaction after the solvent evaporates that eventually turns the whole paint film into a single and hopefully tough molecule) is what defines an "enamel" type paint. What modelers call "acrylics" are technically "water-based enamels", and what modelers call "enamels" are "oil based enamels". "Acrylic" just identifies the material that forms the paint film. Note that across the pond the term "acrylic" sometimes refers to "acrylic lacquer" (automotive touch up paint) - which can lead to all sorts of misunderstandings on the web...

 

I've heard, and seen it happen enough to believe, that adding thinner to a bottle of enamel can cause a mostly full bottle of paint to cure rapidly (a few days) even with the lid tightly sealed. I haven't been scientific enough to know if its all enamels and all types of thinners, but I try to avoid doing that on general principle (I mix the thinner and paint in a separate container).

 

Don

 

I believe you've got it backwards. My evidence is that you can remove enamel using enamel solvents but you can't remove acrylics with anything short of paint stripper once it cures. Try cleaning cured acrylic off of a paint brush.

 

Enamels curing in the jar when you add solvent is more a case of solvent incompatibility. I've seen it much more in acrylics by the way. Put some isopropyl alchohol in Tamiya acrylics and it clots. Put it in Model Master and it's fine.

 

Another possible issue with water based paints is that if they're contaminated, they can be spoiled by mold growth in the paint. Not a widespread problem, but it's possible.

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I believe you've got it backwards. My evidence is that you can remove enamel using enamel solvents but you can't remove acrylics with anything short of paint stripper once it cures.

 

I'm no paint chemist, but I think this is because what we call "model enamels" are fairly low-tech paints: they take a long time to cure completely (weeks) and the material that forms the paint-film (alkyd ?) is not as tough/hard/solvent-resistant as acrylic even after it cures. If you use a modern "oil-based acrylic enamel" - like Krylon spray paint - you'll get a very hard finish that cures in a day or two. Unfortunately you don't have a lot of colors to pick from...

 

I keep hearing about people using the "wrong" thinner with various paints and getting good results - for example using lacquer thinner in Tamiya and Gunze water-based acrylics. There is a semi-famous car modeler (has a how-to-DVD out) who gets amazing results using lacquer thinner with Testor's enamels; he says the lacquer thinner works like a catalyst to make the enamel dry harder. I don't know how the chemistry for this works - I'm guessing modern paint is a lot more complicated than it used to be.

 

Don

Edited by Schmitz

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I keep hearing about people using the "wrong" thinner with various paints and getting good results - for example using lacquer thinner in Tamiya and Gunze water-based acrylics. There is a semi-famous car modeler (has a how-to-DVD out) who gets amazing results using lacquer thinner with Testor's enamels; he says the lacquer thinner works like a catalyst to make the enamel dry harder. I don't know how the chemistry for this works - I'm guessing modern paint is a lot more complicated than it used to be.

 

Don

 

I did some wikipediaeing (is that a word?) after I posted my last post.

 

Enamel is kind of a vague term that basically means "paint that dries with a hard surface". It doesn't really indicate any sort of chemical compostion. Acrylic does though, it is pigment that's carried (emulsified into) an acrylic binder and a carrier (water or petroleum based solvent) which hardens by polymerization and can have either a water base or a solvent base (acrylic laquers). Laquer is a paint with a xylene/tolulene solvent, it dries faster because the solvent evaporates faster than that used in regular oil-based enamels but the pigments and binders may be about the same.

 

I think...

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Acrylics.....while I love the fact, that they are different and SOME cover better than enamels, I have yet to be able to do any cleanup with a water based solvent! If I am SUPER QUICK after airbrushing it, I can ALMOST get it all out of the airbrush! However, I will pull out my laquer thinner to be sure! Future is the worst! Because of it's clarity, there is really no way to check!

 

:Smile_sceptic:

Edited by Mark Aldrich

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Acrylics.....while I love the fact, that they are different and SOME cover better than enamels, I have yet to be able to do any cleanup with a water based solvent! If I am SUPER QUICK after airbrushing it, I can ALMOST get it all out of the airbrush! However, I will pull out my laquer thinner to be sure! Future is the worst! Because of it's clarity, there is really no way to check!

 

:Smile_sceptic:

 

I have a spray bottle of Windex next to the spray booth. The ammonia in Windex cuts right through acrylics if they haven't cured hard. Make sure you clean out the ammonia well though, it will etch into the brass innards of an airbrush after a time. I always finish with a cup of acetone because it is mixable with water so it will clean out any residual moisture and then evaporate out leaving nothing behind.

 

Labs used to use acetone for drying labware, they'd dip it in aceteone and it would air dry in moments. I bet OSHA doesn't allow that any more! It's also fun to throw plastic packing peanuts into a beaker of acetone. They vanish.

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I did some wikipediaeing (is that a word?) after I posted my last post.

 

Enamel is kind of a vague term that basically means "paint that dries with a hard surface".

 

Dave, I was curious about this so I searched a bit too.

 

I grew up in a family of mechanics and auto-body men, so most of my knowledge of paint comes from (full scale) automotive paint from the 70s and 80s. If you go to the Duplicolor (autopaint company) faq you'll see they agree with my definitions of enamel and lacquer. An important difference at the time was that the solvents used in lacquer paint were much stronger than that in enamels - enough so that painting lacquer over enamel paint would melt the enamel and cause it to wrinkle.

 

However as you say, both terms have come down through history with no strict technical definition. I think most modern paints are weird hybrids of what used to be considered "enamels" and "lacquers", which may be why you can get away with using lacquer-thinner (at least in small amounts for thinning) in other types of paint. In my experience most modelers aren't very technical: if its water based paint its "acrylic", if its oil based paint from the hobby shop (Testors or Humbrol) its an "enamel", and any paint with a strong smelling solvent is a "lacquer".

 

Don

Edited by Schmitz

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Sealed correctly paints will last forever, and that's a long time. When your done using the paint, put in a little thinner. Then a little butter on the threads, or on the pop down lid edges. I've got some French Modelcolor paints from the mid 70's that were still good last time I looked. The butter not only will seal, you'll be able to open it next time you want to use that color.

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When I first open a jar of paint, I coat the threads of the jar with a thin layer of Vaseline. It keeps the paint from drying on the threads making it easy to open later and provides a better seal. Another hint to keep paints longer is to only use the manufacturer's suggested thinner. It that's water, fine. If they stipulate distilled water, don't use tap water, it's not distilled. If they say mineral spirits, don't use the new "Green" mineral spirits as they are not the same chemically. If they say use their thinner, it's best to use just that. And finally, if you haven't made yourself a paint stirrer for your Dremel, make one. So much of "saving" old paint is in just stirring it throughly and you may not be able to do that manually.

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