# What makes paint sour?



## Jmayspaint (Mar 26, 2013)

Most of us that have ben in the business for very long have opened a gallon of old paint and smelled that strong, almost soured milk kinda smell. 

We just took over a big job where the previous contractor was using PPG. I opened one of the ceiling paint buckets he’d been working out of and was greeted with that strong sour smell. Didn’t think much of it at the time, thought maybe it had just been a while since he’d been there and that bucket had soured, so I threw it away. 

At this point we’ve been there for a week. I’m not the kinda guy that washes up everything everyday. Seems like too much labor. We cover roller pans, and mini roller pots at the end of the day with damp rags and go right back to it the next. 

What I’ve noticed with this PPG stuff, even using new material, is that it starts to sour within 2 or 3 days when kept like that. The damp rag on top of my mini roller set up will start to stink like rotten milk in just a few days. Don’t see this happening with BM, or Sherwin so fast. 

Does anyone know what chemical causes paint to sour? Maybe one of the techies on here can can help satiate my curiosity about this phenomenon. Nace, Coco, Pac, anybody? 




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## cocomonkeynuts (Apr 14, 2017)

I know sometimes bacteria will feed on certain proteins in paints. I have a couple old cans of iron clad latex in the back that feel pressurized. I opened one and it sounded like a gun shot. Smelled fine and brushed out ok so IDK. Also opened plenty of cans that smell like that classic rotten egg.


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## Local paint pro (May 15, 2018)

I ve noticed it happens to some paints when they freeze or get close to freezing and the chemicals separate. I can tell you from experience that smell doesn’t dry out when the paint dries. I painted half a townhouse And learned the hard way, don’t use sour paint. It won’t be alright!


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## Redux (Oct 27, 2018)

Mold, oxidation of volatile organic compounds, and bacteria are the most common reasons. I work with a lot of vegetable oil based products for wood finishing such as tung oil, linseed oil, and others. The oils become rancid after the can is open and then re-sealed and not used for a while. The best way to preserve your paint would be to get a can of Bloxygen or even compressed CO2. Bloxygen contains argon which is a stable gas and heavier than air. You spray the heavy gas into the can and it settles to the bottom, displacing the air. It prevents mold and aerobic bacteria from growing in the can, mold requiring oxygen to grow. It prevents oxidation of volatile organic compounds. It also prevents paint from skinning over and congealing. An old painter’s trick was to exhale into the can with the lid partially on, then cap it. It worked to a degree, but not as good as the two I mentioned. It's also best to store paint in a cool location. That will inhibit bacterial growth. There’s a video or two on YouTube demonstrating Bloxygen.


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## Woodco (Nov 19, 2016)

Cheap flat paints... Thats what they do. It happens tenfold when there is a roller skin left in the paint.

I dont think its chemical, I think is bacterial, or mold.


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## lilpaintchic (Jul 9, 2014)

Lots of water and not so much good stuff....the more water it has the quicker it gets smelly. Miller paint is NOTORIOUS for that.

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## Redux (Oct 27, 2018)

Chemical reaction for stinky paint:
Carboxymethylcellulose acetate butyrate is used in the manufacture of waterborne coatings. It is combined with acrylic resin to provide film strength, scuff resistance, and weather resistance. CAB can degrade with heat and moisture releasing butyric acid. Butyric acid has a pungent sour smell. Stink bombs are made from butyric acid. That’s probably what you’re smelling.


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## PACman (Oct 24, 2014)

Keeping a good airtight seal will help keep paint from turning "sour". Like alchemy has pointed out, excess moisture in a bucket with acrylic paint will sometimes cause it to smell like spoiled milk. Also, keeping it from temperature fluctuations lessens the chance. As the head space (the air) in a can or bucket expands and contracts with temperature changes, excess moisture is sucked into the container. This excess moisture does two things. It can cause and acceleration of the CAB degradation and can also suck mold and mildew spores into the container. There are additives that control the odor but they add significantly to the cost of the paint, as many of the cheaper additives have been banned by the EPA. If i remember correctly the butyric acid odor should dissipate eventually, but and mold or mildew spores will continue to cause an odor, although a much weaker odor than the acid will cause. If you ever have a room that smells musty no matter what is tried, it is probably mold/mildew contaminated paint. Paint has to be fairly old for this to happen, but the CAB breakdown can happen pretty quickly in new paint in the proper conditions.


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## PACman (Oct 24, 2014)

Or it could just be not enough sugar.


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## Redux (Oct 27, 2018)

Tests have shown the presence of Pseudomonas & Citerobacter genus bacteria in WB paints, the contamination likely being from filtered water in the manufacture process. The bacteria reduce sulfites producing H2S which smells like rotten eggs. Although the bacteria are present, I’m not so certain that sulfites are found in WB coatings or in pigments resulting in H2S production. I’d be leaning more towards the butyric acid theory vs bacterial as the cause for the smell. If I’m not mistaken, glycerin in tint also has a tendency to develop an off, sour/rancid smell. The newer -zero-VOC tints such as Gennex don’t contain glycerin. As PAC mentioned, moisture in paint cans result in CAB degradation and the expansion/contraction of air allowing moisture and mold spores in. Although the argon I mentioned doesn’t have much practical use for preserving WB coatings, it doesn’t expand as much as air due to temperature change, and doesn’t allow for mold/mildew and/or fungal growth. Argon gas in Bloxygen “might “ very well provide a solution for storage of WB finishes.


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## Woodco (Nov 19, 2016)

I dont recall quality paints ever having smell bad. Its always only been crappy flat paints.


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## PACman (Oct 24, 2014)

Woodco said:


> I dont recall quality paints ever having smell bad. Its always only been crappy flat paints.


yup. Porter's cheap flats could knock you on your a55 after a few month in a half empty bucket!


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## deadend (Aug 1, 2013)

...opened some year old Pratt & Lambert a flipper had in an old gallon ice cream container...smelled like an unwashed wild woman...not nice at all...surely wouldn't spread it all over my walls...


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## PACman (Oct 24, 2014)

deadend said:


> ...opened some year old Pratt & Lambert a flipper had in an old gallon ice cream container...smelled like an unwashed wild woman...not nice at all...surely wouldn't spread it all over my walls...


Why the hell would someone store paint in an ice cream container? Don't answer that. I know why.:cursing::wallbash::stupid:


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## deadend (Aug 1, 2013)

...I love that this is what I see instead of emojis....








PACman said:


> deadend said:
> 
> 
> > ...opened some year old Pratt & Lambert a flipper had in an old gallon ice cream container...smelled like an unwashed wild woman...not nice at all...surely wouldn't spread it all over my walls...
> ...


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## PACman (Oct 24, 2014)

deadend said:


> ...I love that this is what I see instead of emojis....
> View attachment 103081
> View attachment 103081


That's even funnier!


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## Jmayspaint (Mar 26, 2013)

Thanks people, I appreciate the detailed answers. I suspected there was a certain chemical in certain paints that caused the phenomenon. 

I can also anecdotally understand the difference in the two possible causes. I have used paint that smelled slightly sour (at my moms house actually) and it dried out fine. No lingering odor. I’ve also heard horror stories about nasty smelling rooms where the smell just won’t go away. There are a few threads on here and at DIY about such situations. 

Am I thinking about it right that there are two possible causes: 
Chemical degradation or mold/mildew growth? I’m imaging the chemical degradation effect possibly drying out and not leaving a lingering odor, and the mold issue possibly being a big lasting problem. 

It’s an academic concern for me at this point in the job as we’ve sold the customer on switching to Aura for the main level of the house. So we’re done using the PPG stuff for now. Still, very interesting to read the technical reasons. 

I too have never experienced this with higher end paints. As mentioned, the old Porter stuff seemed to be particularly susceptible to souring. Seems like some of the cheaper Sherwin lines have done it to me too. It’s never happened with BM as far as I can remember. 


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## chrisn (Jul 15, 2007)

Were you using the blue labeled or the gold?


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## PACman (Oct 24, 2014)

PM 700 and PM 400 are notorious stink bombs!


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