# Exterior Primers



## finishesbykevyn (Apr 14, 2010)

Opinions on acrylic primers for exterior wood. New and weathered. I generally use oil primer on ext. wood , but with technology changing over to water based products, I'm curious on people's thoughts. 
While were on this topic, they have always said to use oil primer on wood, but they also say that oil doesn't breath like acrylics do. So..., how does painting acrylic over an oil primer make the whole system breathable..hmmm.:whistling2:


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## MikeCalifornia (Aug 26, 2012)

Who is they? All products have a perm rating. Perms are the measure of moisture vapor that can pass through a coating. Oil primers and acrylics are compatible, but acrylic topcoats over old oil topcoats are not advisable.


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## MIZZOU (Nov 18, 2012)

Soon enough oil primers may not even exist. I've used 123 on certain exteriors for years and never had a problem. new wood I always use oil.


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## CApainter (Jun 29, 2007)

MikeCalifornia said:


> Who is they? All products have a perm rating. Perms are the measure of moisture vapor that can pass through a coating. Oil primers and acrylics are compatible, but acrylic topcoats over old oil topcoats are not advisable.


Where do you find the perm ratings? I looked at the following product TDS's and couldn't find perm ratings:

-BM Fresh Start Alkyd Ext. Primer
-PPG Pitt Tech Plus WB DTM
-Zinsser Cover Stain


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## Gough (Nov 24, 2010)

CApainter said:


> Where do you find the perm ratings? I looked at the following product TDS's and couldn't find perm ratings:
> 
> -BM Fresh Start Alkyd Ext. Primer
> -PPG Pitt Tech Plus WB DTM
> -Zinsser Cover Stain


http://www.benjaminmoore.com/Downlo...Id=BEA+Repository/16460/data_sheet_file_en_US

For the other two, I think you have to look a little farther.


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## CApainter (Jun 29, 2007)

I completely over looked that. Thanks Gough.


"• Possesses a “perm”* 
rating of less than 1; 
effectively performs as a 
vapor barrier, sealing in 
moisture that would 
otherwise pass through 
outer walls and ceilings 
under unheated attics" 

So is this a good or bad thing? I'm not getting this. Please help!


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## Red dog (Jul 20, 2014)

I like oil based exterior primers on any bare wood whether I'm spot priming or new wood.I know there are good acrylics but I was taught to prime exterior wood with oil and I'm too old to change now lol.


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## Gough (Nov 24, 2010)

CApainter said:


> I completely over looked that. Thanks Gough.
> 
> 
> "• Possesses a “perm”*
> ...


It's not necessarily good or bad, it depends on what you it to do. If you're trying to slow water vapor migration through (exterior) GWB walls, its a good thing. If you're trying to allow drying to the outside in a poorly-designed (non-rainscreen) wall system, it's not such a good thing.


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## CApainter (Jun 29, 2007)

Other then having an understanding that warm air carries moisture and migrates to colder areas, my knowledge of a structure's envelope and how it relates to moisture permeability and thermodynamics, is limited.

I believe that protecting against any moisture from entering a building's envelope is very important, and that a vapor barrier will help with that. However, I also understand that if a vapor barrier does not seal 100% of the envelope, moisture has the potential to enter, with only a limited chance of escaping. Resulting in trapped moisture, and the subsequent affects of blistering vapor barrier coatings as it is trying to escape. This has always been a concern of mine, and one of the reasons I prefer a more permeable waterborne coating system. Particularly with standard, wood siding construction. Where I do tend to lean more towards oil base primers, is on exterior plywoods like T1-11. 

I'm sure interior air circulation plays a big part in moisture control, but there are so many variables with that.


My expertise has become less reliable on residential homes since I've had limited experience with them in the last fifteen years. In terms of coating permeability, since most of my projects have steel substrates, it is pretty simple to pick the coating with the least permeability that will also build the best moisture barrier. However, when it comes to galvanized sheet metal that requires coating flexibility, Alkyd based barriers aren't the best choice either.


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## Painter-Aaron (Jan 17, 2013)

This topic comes at a good time.

Iv always been handed cover stain working for somebody else 

Now I read it gets brittle and not so good for priming large exterior areas 

My first year in business And last year iv been mainly inside and no full exteriors. Only trim packages and decks 

Now I have a few full exteriors lined up and one of them being a heritage house. 

Iv noticed on areas where bubbling was previously present, if I scrape sand and prime and paint with a latex primer the top coat will be solid where it was spot primed over bare wood but pull up and bubble where there was previous coats. 

Iv bought a gallon of the long dry oil for old stuff like that to see if that might be better

For this heritage house a large portion is bubbling and when you scrape it off it looks glossy or like an old stain just before the wood. And then there is a bunch of new siding. 

My plan was to apply long oil over the old sections after scraping and sanding and using latex over the new wood as it's easier to use. 

But reading this discussion it says oil is better over new wood. Would anyone suggest just using the long oil everywhere?


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## Gough (Nov 24, 2010)

CApainter said:


> Other then having an understanding that warm air carries moisture and migrates to colder areas, my knowledge of a structure's envelope and how it relates to moisture permeability and thermodynamics, is limited.
> 
> I believe that protecting against any moisture from entering a building's envelope is very important, and that a vapor barrier will help with that. However, I also understand that if a vapor barrier does not seal 100% of the envelope, moisture has the potential to enter, with only a limited chance of escaping. Resulting in trapped moisture, and the subsequent affects of blistering vapor barrier coatings as it is trying to escape. This has always been a concern of mine, and one of the reasons I prefer a more permeable waterborne coating system. Particularly with standard, wood siding construction. Where I do tend to lean more towards oil base primers, is on exterior plywoods like T1-11.
> 
> ...


The short answer is that, in heating climates, the vapor retarder should be on the inside and on the outside in cooling climates. With a ventilated rainscreen system, both interior and exterior surfaces can be vapor impermeable, since the rainscreen allows the wall to dry.

Coatings on exterior plywoods, like T1-11, will last substantially longer when they are primed with a water-borne sysyem. The brittle nature of the oil primers leads to widespread cracking, water infiltration, and delamination. With T1-11, there is the additional problem of water intrusion via the exposed end grain in the grooves. That leads to trapped moisture and more damage.


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## Gough (Nov 24, 2010)

Painter-Aaron said:


> This topic comes at a good time.
> 
> Iv always been handed cover stain working for somebody else
> 
> ...


For new high-tannin woods, we use long-oil. 

I think one detail that gets overlooked is making sure that it's a thin coat of oil primer that's brushed on.


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## finishesbykevyn (Apr 14, 2010)

Great discussion. Without getting too tech Its definately better to prime bare wood with something rather than nothing. Depending on how the house was built, breathability, proper water flow etc. Your only getting 5-10 years on a paint job anyways. Give or take i guess... proper washing and prep is definately the most important!! So many variables indeed..

Sent from my SGH-I747M using PaintTalk.com mobile app


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## Painter-Aaron (Jan 17, 2013)

Gough said:


> For new high-tannin woods, we use long-oil.
> 
> 
> 
> I think one detail that gets overlooked is making sure that it's a thin coat of oil primer that's brushed on.



Do you think a long oil might help reduce the risk of the new paint pulling up the old paint with bubbles?


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## Gough (Nov 24, 2010)

Painter-Aaron said:


> Do you think a long oil might help reduce the risk of the new paint pulling up the old paint with bubbles?


Good question...I wish I had a good answer. I think I could come up with a cogent argument either way, but I really don't know. One of the things we've always tried to do was remove as much of the failing paint as possible, and I can't recall a situation where we've painted over bubbled paint.


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## Painter-Aaron (Jan 17, 2013)

Gough said:


> Good question...I wish I had a good answer. I think I could come up with a cogent argument either way, but I really don't know. One of the things we've always tried to do was remove as much of the failing paint as possible, and I can't recall a situation where we've painted over bubbled paint.


your right, the answer is to strip as much as possible. But I am not saying I am painting over bubbling paint, as much as I noticed areas that have bubbles present prior to starting usually have problems afterwards. 

For example you have a section of siding that has bubbles, I scrape, sand as much off and feather off what is solid, prime it, and then paint over it. 

The areas where I sanded down to bare wood and primed are always good but any paint over where the paint wasn't removed, but was solid enough to not have to sand starts to bubble. 

Sometimes its just not in the budget to remove all the paint, butI cant just let it bubble up. 

I haven't tried the long oil yet, I have a gallon but I was hoping maybe that might help a bit with this problem.


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