# Can I use interior latex as a primer for exterior paint?



## Verity (Sep 4, 2009)

Hi all, I'm new here.
We own a small rental house about 2 hours away.  It was built in 1908 but we don't know if the garage/shop was built then or (possibly) as late as the early 40's.  The garage has cedar shingle siding and has apparently never been painted, it's in rough shape and it's our next project. We haven't been able to find shakes/shingles that match the originals in any way except the fact they are made from the same wood so using the best match possible we'll patch what's missing and then we're going to paint it to match the house - a pale yellow, white trim, red doors.
My real question how will the final paint job be effected if I use some of the many buckets of interior latex we already have as a base coat for the 'real' coat of exterior paint.  Unpainted, weathered cedar soaks up paint like crazy and we just can't afford to buy gallons and gallons of paint.  I have only 3 gallons of stain blocker (Zinsser brand I think) and know from experience it won't cover even one side of this small garage.
My plan is to use any and all of my left-over latex - can by can, not mixed into a big bucket (they are all brands and levels of gloss so they won't mix) - until the shingles seem to be pretty well covered.  I was going to start using the darkest color and flattest finish first on the theory that the next layer of paint would stick better to a flatter finish. The Zinsser is white and sticks to anything so my hope is that it will act as 'glue' to hold the final coat.
The garage needs to look "better" it doesn't have to look fabulous. I'll be doing all of the priming while my dh is dealing with other repairs and had planned on using a brush but should I use a pot sprayer instead?
The house is in one of the dampest areas of the NW if that means anything to anyone.


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## Nestor_Kelebay (Sep 7, 2009)

Well, I don't see a LOT of major problems doing what you're thinking, but I will outline what the differences between interior and exterior primers and paints are.

I'm a firm believer that oil based paints outlast latex paint over wood outdoors, so in a perfect world, I'd like to see you using an oil based primer over the bare wood, and top coating with an oil based paint.

Lemme educate you on what you're doing, and why it'll work "ok", but won't work as well as using an exterior oil based primer on the bare wood and top coating with an exterior oil based paint.

First, the theory...

Wood is a natural material and stretches and shrinks with changes in the seasonal temperature and humidity.  That's not to say that wood has any significant amount of thermal expansion with temperature associated with it; it doesn't.  But, as both the outdoor temperature and humidity increase, wood absorbs more and more humidity from the air, and the result is that this humidity goes into the wood cell walls, causing them to swell up.  Wood cells are shaped like long drinking straws, but they're closed off at the ends.  Because of the shape of wood cells, there are very many more wood cell walls going across the grain than along the grain, and the result is that wood swells much more across it's grain when it absorbs humidity from the air than it does along it's grain.

Now, both interior and exterior LATEX PAINTS have more than enough elasticity to stretch and shrink with the wood so that neither one will crack and peel if used outdoors over bare wood.  Basically, the only difference between interior and exterior latex paints is the amount of additives in them.  Exterior latex paints will have UV blockers and more fungicides added to them to prevent molds growing on the paint in humid conditionns.  Also, exterior latex paints will tend to dry to a softer film than their interior latex counter parts, but the reason why isn't because they need to be softer in order to stretch and shrink with wood.  It's because interior latex paints have to be harder to sand up to hard scrubbing to remove stubborn marks on walls.  Consequently, paint companies will use a less expensive binder on their exterior latex paints simply because no one is every going to scrub hard on them to clean them as you would a latex paint.  (Although, that might not be true anymore with kids spray painting graffiti all over the place.)

If you take away the need for UV blockers and fungicides to protect the paint from the Sun and attack by fungii in humid environments, there wouldn't be much difference between interior and exterior latex paints.  And, so if you use an exterior latex paint with those UV blockers and extra fungicide in it to top coat a primer (to protect it from the Sun and prevent fungii from growing on that primer), then you don't need the UV blockers or extra fungicide in latex primers.  Consequently, there really is NO difference between interior and exterior latex primer.  If you find a latex primer claiming to be "for interior use" or "for exterior use", the difference will be that one smells less as it dries than the other.  There would be very little difference in the dry primer film left behind.

It's in the EXTERIOR oil based primers and paints that there is a much more important difference when it comes to painting wood outdoors.  Basically, hardness is an advantage when it comes to paints.  The harder a film a paint forms, the less quickly it will be marked up by things rubbing against it, and the better it will stand up to everyday wear and tear.  Oil based primers and paints form much harder films than latex coatings do.  And, with hardness comes rigidity, and that means oil based primers and paints simply can't stretch as far as wood outdoors will.

To allow oil based primers and paints to be used over wood outdoors, they've had to weaken EXTERIOR oil based primers and paints so they dry to a softer film that will stretch and shrink with wood outdoors.  That is, the fundamental difference between interior and exterior oil based primers and paints is that exterior oil based primers and paints are formulated so that they don't dry to as hard a film as they otherwise wood.  And, the purpose in keeping them softer is so that they retain enough elasticity to stretch and shrink with the wood they're applied over when used outdoors.

Exterior oil based PAINTS have UV blockers and more mildewcides in them than interior paints, just like you have in latex paints.  However, where you don't need those additives, such as in the primer which is protected from the Sun and fungii by the top coat paint, the big difference between interior and exterior oil based primers is the hardness of the film they dry to.  An interior oil based primer or paint will peel off wood outdoors in a few years.  An exterior oil based primer or paint will last longer than grandma on wood outdoors.

But, notice that all we seem to be talking about is wood.  That's because the amount wood expands and contracts outdoors absolutely dwarfs any thermal expansion or contraction you're gonna come across due to temperature change alone.  It's not at all unusual to see a 5 or 6 percent expansion in wood outdoors due to it's changing moisture content.  However the thermal coefficient of expansion for common materials like wood, iron or concrete are of the order of 5 to 20 millionths of a percent per degree Celsius.  So, if you go from -40 deg. C below in the winter (which you can expect during a self respecting winter in Winnipeg) to +40 deg. C. in a self respecting Winnipeg heat wave, you have an 80 deg. C temperature spread.  80 degrees X 20 E-6 is about 0.16 percent.  So, thermal expansion of materials is TINY compared to the swelling of wood outdoors due to changes in it's moisture content.

So, don't be scared to use an INTERIOR oil based paint outdoors if you're painting over steel or concrete (if you don't live in a southern latitude where you need UV blockers cuz of the Sun, and you don't live on a shoreline where you need fungicides to protect the paint from mold growing on it).  That's because even the hardest interior oil based paints have enough elasticity in them to accomodate thermal expansion of common building materials.  It's wood that's the problem, and not thermal expansion of wood; moisture expansion of wood.


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## Nestor_Kelebay (Sep 7, 2009)

This web site won't let me post a response longer than 10,000 characters, so I had to break it up into two posts.

Here's the second half:


Now, for the practical application of the theory...

So, what you're essentially doing is using an interior latex paint as an exterior latex primer.  I don't see much problem with that.  You don't need the higher hardness of interior latex paints over exterior latex paints, but you'll still have plenty of elasticity with your interior latex paint to stay stuck to wood outdoors.  There won't be a problen there.  You don't need UV blockers or as much fungicide in the paint if it's gonna be top coated with a paint that does have that stuff in it, so not having that stuff in the interior latex paints won't pose a problem either.

What I am concerned about is:

0. Painting over deteriorated wood.  I'd use a paint scraper or sander to sand off the grey fibers at the surface of the wood to get down to undeteriorated wood before you apply your latex paints as a primer.

1. Painting over a gloss latex paint with your exterior paint.  You may have a problem with adhesion to a smoother surface, even if you're using an exterior oil based paint over a latex.   The exerior oil based paint won't stick to that smooth latex paint as well as you might think.  If you use any semi-gloss or gloss latex paint as a primer, then wait for it to dry and sand it down to roughen the surface before top coating with your light yellow paint.

2. Tanning staining of the latex paints.  Tannin is a brown chemical that's soluble in water.  Cedar has a lot of tannin in it, and one of the problems with using a latex primer over cedar is having the primer acquire a brownish discolouration.  That's caused by tannin from the cedar dissolving in the water of the primer.  You'll notice this less with a coloured paint than a white primer, but if you see that, now you know why.  Tannin isn't soluble in mineral spirits, so you have much less tannin staining if you use an oil based primer.  But, by the same token, if you use an oil based primer over your discoloured latex paint/primer, then the tanning shouldn't bleed through the top coat.  (It prolly will bleed through another latex top coat, tho.)

3. The colours you're going with aren't great for hide.  The yellow and red pigments typically used in house paints are Arylide Yellow and Napthol Red.  Both of these pigments have lousy hide, and aren't very colourfast.  They tend to fade from the UV light from the Sun faster than other pigments.  But, Seattle is a more northernly latitude, so hopefully the fade won't be too much.  (You'll learn how much once you look at the new paint on your garage versus the old paint on your house.)  The saving grace is that a PALE yellow will have a lot of white pigment in it.  If you buy a "budget priced" paint, that white pigment will be chaulk (calcium carbonate) or talc (magnesium silicate), and those white pigments don't hide well either.  To get good hide in a light coloured paint, you pretty well need to pay more and buy a higher quality paint where the white pigment is titanium dioxide.  Titanium dioxide is what paint companies turned to when they banned lead carbonate as the white pigment in paints back in the 1970's.

The red pigment in house paints is typically Napthol Red or Toluide Red.  Both have poor hide and fade from UV light a lot.  So, I'd be concerned about how many coats of pale yellow and red paint it's going to take to hide any underlying latex paint colours.  I'd probably plan for at least two coats, and maybe 3 or 4.

Some people would be concerned that using an exterior oil based paint over the garage will cause the paint to gradually yellow in colour like it does outdoors.  But, luckily only people that know nothing about paint would be concerned about that.  Yellowing of oil based paints is something that happens under low lighting conditions.  Outdoors, or even indoors where you have plenty of natural lighting, yellowing doesn't occur in oil based paints.  Rather, it's probably more accurate to say that the UV light from the Sun bleaches out that yellowing faster than it occurs, so you'll never see oil based paints yellow outdoors (or even indoors where there's plenty of natural light from windows and through doors).  In fact, if you take something that's got yellowed oil based paint on it outdoors, you'll find that the yellow discolouration disappears in a few weeks.  (It'll start gradually turning yellow again as soon as you put it back into it's dim lighting condition again, tho.

Don't know how much this helps.


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## Verity (Sep 22, 2009)

Thank you for all the information! 
Because of a computer crash I didn't get it prior to my painting trip but I decided to just "go for it."  I was extremely surprised to discover the shingles soaked up much less paint than expected.  Perhaps it was due to the moderate temperatures. 
When I actually inventoried all the interior paint available I discovered I had so little dark and so much in light colors I just used the light colors. I have the 1st coat on (to the cheers and thumbs up from neighbors!) and will apply the second coat over the next couple of weekends. 
Since you seem to be a paint expert perhaps you could explain what's happening to the paint on the house itself?  We spent weeks prepping the house using scrapers, flapper wheel thingies on drills, sanders and I don't know what-all.  In some places the old paint had blistered and lifted off cleanly leaving only a faint stain on the wood, in other places (sometimes only inches away) the paint layers adhered as if super glued and no amount of scraping or sanding would budge it.  I was against using heat because of the lead content in the old paint but now I'm not so sure I was right.  
To be honest, we gave up in disgust and finally slapped a coat of paint on the house. Here's the problem: in the places where the old paint would not release it is now blistering but ONLY on one side of the house.  It's the west side but literally almost never gets any sun as a larger (taller and longer) house is next door and the two houses are less than 4 feet apart. 
Why would the old paint continue to blister on this side of the house only?  The east side of the house has just as much area that is new paint over old but the neighboring house on that side is about 10 feet away. Neither the front (south facing) or back (north facing) are showing any blistering either.
My current theory for repair is to simply scrape any blisters and slap on some more paint in the hope that eventually all the old paint on the east side will eventually eliminate itself. (Since we used all the yellow on the house last year I peeled a chunk of the blistered paint and will have it color matched so I won't see the fading you warned me to expect!)
Thanks again for the extensive information,
Verity


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## allpaintersRdrunks (May 2, 2013)

first, don't listen to whomever he is, second, acrylic is not the best primer over cedar, cedar is a b...tch to get adhesion to, it has oils in it, that can both cause paint not to adhere, which may be why you had that problem of the spots that it fell off of, if you want to use your acrylic(s), MIX it all together, gloss/flat doesn't matter, it will come out satin, and add some FLOOD Emulsibond, and maybe throw a little penetrol in it, FLOOD EB used to include penetrol in it, but they took it out.  that will help your adhesion in your initial coat, Use any acrylic primer you got, too.


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## nealtw (May 3, 2013)

Thanks for the input, but this is really old, likely no one cares anymore.


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## justacook (Jul 11, 2013)

Thank you very much for your info on indoor out door primer. I kinda had a hunch that there was not a whole lot of difference between them, however, Im just a cook, but my father taught  me 2 things, if nothing else. 1. No matter how simple it seems to do, read the instructions first...It will save a lot of time later, and 2. the wisest man in the world does not know everything, but he does know where to go to get the information he needs....thanks again


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