# Mistake Correction: Interior vs. Exterior Paint



## ejdcohen (May 15, 2009)

I saw a reply by Nestor Kelebay about _not_ using _interior_ paint for the walls of a three-season porch, AFTER I had primed my porch with two coats of interior primer!

What should I do now?  Do I prime again with a exterior primer? I also primed the ceiling. Not sure if it matters, but I live in Chicago, IL.Thanks!


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## Nestor_Kelebay (May 15, 2009)

Did you use an interior LATEX primer or an interior ALKYD (oil based) primer on your porch?

Are the surfaces you primed wood or some other material?


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## Nestor_Kelebay (May 17, 2009)

I should explain to people that there's a very big difference between interior and exterior alkyd primers, but virtually no difference between interior and exterior latex primers.

Because wood outdoors swells and shrinks with seasonal changes in temperature and humidity, and alkyd primers and paints dry to too hard a film to stretch and shrink with that wood, exterior alkyd primers and paints will be formulated to dry to a softer film that can stretch and shrink without breaking.  Also, since alkyds form an impermeable film, exterior alkyd primers will often have more than the "CPVC" or "Critical Pigment Volume Content".  If the volume of pigment in the can is below the CPVC, then the primer will be impermeable to air, water and humidity.  Once you get above the CPVC, then as the primer dries and shrinks, pores open up between the extender pigments, and the primer essentially dries porous.  The purpose in putting so much extender pigment in exterior alkyd primers is to allow moisture to evaporate out of the substrate through the primer.  Typically, tho, you'd still have to use a latex paint over top of that exterior alkyd primer to allow the H20 molecules to escape through the paint film.   If you use an exterior alkyd paint over that exterior alkyd primer, then the primer will allow air and humidity to pass through it, but not the alkyd paint, so the humidity in the substrate would still be trapped.

Both interior and exterior latex primers and paints are plenty soft enough to stretch and shrink with wood outdoors.  And, both will allow H2O molecules to pass through them relatively easily.  Typically latex primers will advertise themselves as being 
"Interior/Exterior".  If you do come across a latex primer that says it's for Interior use or Exterior use, the difference is typically just the type and amount of coalescing solvent that evaporates from it.  The more and smellier the coalescing solvent that evaporates from it, the greater the chances that it'll be sold for exterior use.  Typically, that smell is gone in a coupla days, so if you're not sensitive to that smell and get headaches from it, don't hesitate to use "exterior latex primers" indoors, and "interior latex primers" outdoors.

Also, since the difference in hardness between interior and exterior alkyd primers and paints is entirely due to the swelling and shrinkage of wood, you can use interior alkyd coatings outdoors over other substrates that don't swell and shrink like wood.  Metals and concrete have SOME thermal expansion and contraction, but it's absolutely negligible compared to the amount wood swells and shrinks.  So, if you don't live in the south where UV inhibitors in the paint are important, or near an ocean or lake where humidity and mold and mildew growth are a problem, you can use interior alkyd primers and paints over bare metal outdoors, such as on a steel garage door, or over stucco outside.


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## ejdcohen (May 17, 2009)

Thanks for the awesome, and complete reply!  I primed the room with an exterior latex primer prior to reading your post.  Hopefully everything will be fine over the coming winters.


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## Nestor_Kelebay (May 18, 2009)

Good.

I was concerned that you'd use an interior alkyd primer over a bunch of wood outdoors and I'd have to tell you the only thing you can do is wait for it to crack and peel to make it that much easier to scrape off.


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## fuji0030 (May 21, 2009)

Both autogenous and exoteric acrylic primers and paints are affluence bendable abundant to amplitude and compress with copse outdoors. And, both will acquiesce H2O molecules to canyon through them almost easily. Typically acrylic primers will acquaint themselves as being

"Interior/Exterior".


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## cecilgrass (Aug 26, 2010)

Great tips thanks for sharing. I was lucky that the last time I needed parts of my house painted and I had to find <a href="http://www.rainerpainting.com/">Moorestown NJ painters</a>, the guys at Moorestown NJ Painters | South Jersey Painting Contractors | Haddonfield NJ Painters said something similar. Many times I've hired out or done it myself and never did I know this. If only I knew this 10 or so years ago.


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## drewpy (Aug 31, 2010)

There are only a few "rules" with primer. New wood should be primed with oil, oil-based paint needs to be primed with oil primer if you're going to paint latex on top of it, and new masonry should be primed with latex. Other random notes include shellac based primer for fire damage and painting on top of wallpaper, and either oil or shellac for covering knots in wood.


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## Nestor_Kelebay (Aug 31, 2010)

Drewpy is approximately 1/2 correct.  Lemme explain:

New wood should be primed with oil.
      That's if you can find a linseed oil based primer anymore.  You see, linseed oil based primers penetrated deeper into the wood because oil molecules are tiny compared to alkyd resins, and therefore can penetrate deeper into the wood than alkyd resins.  In fact, one of the reasons why the old linseed oil primers were thinned with the slower drying turpentine was to allow for a longer drying time, and therefore deeper penetration into the wood, and therefore better adhesion to the wood.  Nowadays, however, you can only get interior alkyd primers, and alkyd resins are so much larger than oil molecules that they don't penetrate as deeply.  So, alkyd primers are thinned with the faster drying mineral spirits because they don't penetrate very deeply into the wood anyway because of their larger size.  Thus, thinning alkyd primer with turpentine would only increase the drying time without increasing either penetration or adhesion.

Still, alkyd primer penetrate and adhere better to wood than latex primers which are vastly larger still, so using an alkyd primer on wood is better than a latex primer.

Generally, if you want to paint over oil based paint with latex paint, you can either sand the old oil based paint down, or paint over it with an alkyd primer which will both stick well to an alkyd paint and dry rough enough for a latex top coat to stick well to it.

"...new masonary should be primed with latex."
     New concrete (less than two years old) is still highly alkaline.  That's because they make concrete with hydrated lime, which is Ca(OH)2 or  HO-Ca-OH.  Over the first two years of that new concrete's life, the hydrated lime, Ca(OH)2, reacts with the CO2 in the air to form limestone, or calcium carbonate, CaCO3.  As that happens, the alkalinity of the concrete subsides.
     If you want to paint over new concrete within the first 2 years of it's life, then you need to use a special acrylic primer made specifically for new concete.  Such a primer will be made from polymethyl methacrylate resins that are chosen specifically for their high alkaline resistance.  Not all latex primers are made with PMMA resins.  A general purpose latex primer for wood or drywall will typically be a "PVA" primer, meaning it is made with polyvinyl acetate resins, and won't have the high alkaline resistance you need over new concrete.

     If the concrete is more than 5 years old, it's no longer alkaline, and you can prime it with either oil or latex primers.  However, concrete is rough enough that it really doesn't need to be primed, and you can paint directly over it with your paint if you want.

...and either oil or shellac for covering knots in wood.

     The reason why knots in wood will often "bleed through" a latex primer is because knots contain a lot of "tannin" or tannic acid.  Tannic acid is a brown material that's highly soluble in water.  So, if you use a latex primer over bare wood, the knots will often bleed through because the tannin in them dissolves in the water of the primer.  Tannic acid isn't soluble in either mineral spirits or alcohol, and so it won't dissolve in either an alkyd or shellac based primer, and therefore won't "bleed through" oil based or shellac based primers.


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