exterior priming/painting questions


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Old 05-08-11, 09:58 AM
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exterior priming/painting questions

I've recently hardi-planked my house myself. I believe the plank is pre-primed, but, wanted to check with others here before going any further.

Should I add another coat of primer? Anything in particular I should look out for when buying exterior paint?

Also, how do you prime treated wood and any special type of paint?


Thanks all!
 
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Old 05-08-11, 11:04 AM
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Texas Gun Owner:
You don't always need to prime, but priming won't do any harm. Rough surfaces like concrete and stucco don't need to be primed. The surface is rough enough for paint to adhere well to stucco or concrete without priming first. (You can buy special latex concrete primers, but these are intended to be used over FRESH concrete where the high alkalinity of the new concrete would damage any paint applied directly to concrete.)

So, if this siding is made out of the same stuff as Hardibacker board, I'd say to paint right over it. I think that Hardibacker is homogenious enough that you shouldn't have to worry about the paint soaking into the siding more in some places than others.

I think the company that made your siding probably means that it doesn't need to be primed rather than it comes pre-primed from the factory as I don't see any benefit in priming a rough enough surface.

As with anything else, buy a top quality exterior paint, whether latex or oil. In my opinion, oil based paints simply stand up better outdoors than latex paints. Also, it's true that oil base paints yellow with age indoors where there isn't plenty of natural sunlight. Oil based paints exposed to plenty of direct or indirect natural sunlight won't yellow with age.

The only consideration when painting pressure treated wood is to give it about two years before priming or painting. That's because pressure treating involves pumping the water based treating solution into the wood under pressure. You need to allow time for the water to evaporate from the wood before priming or painting. That is, any wood has to be dry enough that the evaporation of water from the wood doesn't push the primer and paint layer off. The pressure treatment chemical really doesn't come into the picture because it remains INSIDE the wood, not on the surface where the paint is.
 

Last edited by Nestor; 05-08-11 at 12:08 PM.
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Old 05-08-11, 02:49 PM
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I've never been overly fond of or reliant on factory primers, IMO it's best to reprime the siding. Oil base primer/paint is not recommended on hardi plank siding. A quality latex house paint should be used. You'll find the best coatings at your local paint store - not a big box paint dept. Price is usually indicative of quality.

PT wood can be tricky to paint as it should have at least 6 months drying time before applying a coating. Since this isn't always practical, it's best to use latex primer and paint as it will breathe some. When feasible I prefer to use a latex solid stain on the PT wood instead of paint.
 
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Old 05-08-11, 04:19 PM
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Texas Gun Owner:

On this web page:
James Hardie: Painting Tips | Residential Siding from James Hardie
the manufacturer recommends their own primer called Prime Plus to be used over your siding. Apparantly, this primer is the first ever to be formulated to be used over fiber cement.

They also recommend against using oil based paints directly over unprimed fiber cement because they say it may increase surface roughness, cause loss of adhesion, cracking and excessive chaulking of the paint. But, nowhere do they say not to use an oil based paint over their Prime Plus primer.

There's a list of paints they recommend on that web page, all of which appear to be latex paints. When you see the wording "100% Acrylic" in that list, it only means that the plastic resins which coalesce to form the paint film are made of a plastic called "polymethyl methacrylate" which is the same plastic that Plexiglas is made of. EVERY paint that says it's either "100% Acrylic" or "vinyl acrylic" will be a latex paint. Oil based paints don't use these kinds of plastics in their binders.

Don't presume that all paints that claim to be 100% Acrylic are equal in quality. There are hundreds of different resins, each with different characteristics in terms of hardness, clarity, adhesion and chemical resistance, all of which are made from polymethyl methacrylate. And, depending on what characteristics are important in each product, these resins are are used to make everything from latex paints, acrylic floor "waxes", grout and masonary sealers and fingernail polish for the ladies.

So keep in mind that there are very many kinds of resins made from polymethyl methacrylate, all of which are "100% Acrylic", and all of which have different properties. In fact, my experience has been that it's the lesser quality paints that will advertise that they're "100% Acrylic" to impress potential consumers. In reality that wording on a can of paint means about as much as the wording "100% Cow" does on a package of meat. It doesn't say if you're getting prime rib or dog food or anything in between. Price is a better indicator of quality when it comes to paint than the wording on the can.

Also, if it wuz me, I would paint with a DEAD FLAT paint. That way, years from now when it comes time to repaint your siding, you won't have to scuff sand the gloss off your old paint for the new coat to stick better. You'll already have a rough surface that the new paint should stick well to.

PS:
I'll bet dollars to donuts that this Prime Plus primer is nothing more than an acrylic primer meant for fresh concrete which you can buy at any paint store. You see, there's something called the Limestone Cycle, whereby calcium forms different compounds:

What is Lime? | Products | Graymont

To make concrete you combine portland cement, hydrated lime and aggregate with water. Hydrated lime is calcium hydroxide or Ca(OH)2, and it's those OH groups in the fresh cement which makes it highly alkaline (or gives it a high pH). Oil based primers and paints can't be applied over fresh concrete because of a chemical process called "saponification" which literally converts the oil in the primer or paint into soap. (This is the same process by which oven cleaners convert the baked on grease in an oven into soap so that it can be more easily removed since soap is soluble in water.) Only certain kinds of acrylic resins are alkaline resistant enough that they can be used to make primers that will stand up on fresh concrete, and even then, only after he concrete has been neutralized with an acid wash or after it's cured for at least 2 to 4 weeks.

Once on, that acrylic primer forms a physical barrier between the alkaline concrete and whatever top coat of paint you use over the primer, so that that the still highly alkaline concrete can then be painted to finish the project.

Priming and Painting Concrete and Masonry

Over time, the Ca(OH)2 in the fresh concrete reacts with CO2 in the air to form calcium carbonate, or CaCO3 and a molecule of water which turns into a butterfly and flies away. But, over the course of one or two years, as that Ca(OH)2 disappears and calcium carbonate forms, the alkalinity of the fresh concrete subsides (because of the loss of those OH groups) to the point where it can be painted over with any paint, latex or oil.

what is saponification?

James Harie is as secretive as any other company about it's proprietary products. But, since they make their siding from cement, that siding is just as alkaline as fresh concrete is, and I fully suspect their Prime Plus primer is the same as any acrylic primer made specifically to be used over fresh concrete. And, just as with fresh concrete, if you were to wait 2 or 3 years before painting your siding, you could use whatever paint you wanted without concern (even though James Hardie recommends you prime and paint your siding soon after installing it).
 

Last edited by Nestor; 05-08-11 at 05:24 PM.
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Old 05-28-11, 09:25 AM
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Thanks guys ... now the fun begins.
 
 

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