Bwildly

I am going to paint my exterior foundation "poured concrete". I have bought self priming masonary paint but was wondering if I should still use a primer first? I have properly preped the surface for painting by scraping, power washing, and acid washing the surface but looks like its going to be a week or longer before I can do anything because of the weather. Thanks in advance for any help you can give me.

Nestor_Kelebay

Never paint concrete unless it's at least two years old. The reason why is that fresh concrete contains a lot of hydrated lime or Ca(OH)2 or HO-Ca-OH and it's those -OH groups in the fresh concrete that make it so alkaline that only certain kinds of coatings will tolerate it.

This web page explains why concrete and real plaster gradually become less alkaline during the first two years:
What is Lime? | Products | Graymont

It takes AT LEAST a year for the CO2 in the air to turn that hydrated lime into limestone, with a resulting lowering of the very high pH of the concrete, thereby making it more neutral.

No, never use any primer under masonary paint, nor use any paint over it except another latex masonary paint.

Masonary paints are latex paints formulated specifically for exterior masonary (brick and block, normally) walls. When water gets into brick or other masonary walls, it has little deleterious effect on the brick (or other masonary) unless that water freezes. If it does freeze, the pressure of the expanding ice inside the masonary units as the water freezes can do considerable damage.

Masonary paints are formulated from from latex paint resins which can be thought of for the purposes of this explanation as long wires scrunched into balls. Even though the wire is scrunched up, there are still gaps between various segments of wire inside each ball. Masonary paints are made from latex paint resins where the distance between those various wire segments in each ball are larger than a single H2O molecule but smaller than the distance between H2O molecules in liquid water. So, only single H2O molecules can pass through a masonary paint film, not liquid water.

That means humidity can pass in either direction through a masonary paint film, but liquid water cannot pass in either direction through it, and so moisture inside the wall can evaporate through the masonary paint, but the paint will stop rainfall from outdoors from getting into the wall. In this way, masonary paint help keeps walls dry to protect them from the effects of freezing water.

Using a primer under masonary paint, or painting over it with anything but another masonary paint will defeat the purpose of using a masonary paint in the first place.

You don't need to go through all that to get paint to stick to concrete. Unless it's polished concrete (like Costco has on their store floors) any paint will stick well to bare concrete without a primer, and with no more preparation than to ensure the concrete is reasonably clean.

The only time you need to prime concrete is if it's less than a year old. In that case you need to use an acrylic primer made specifically for this purpose. That primer acts as a physical barrier between whatever paint you use over it, and the highly alkaline concrete under it.

PS: You don't need to know this:
The reason why true drying oil based paints (like linseed oil based paints) won't last on fresh concrete is because of a chemical process called "saponification". This is the process whereby an animal fat or vegetable oil reacts with a strong alkali to make soap. So, if you paint over fresh concrete with a linseed oil based paint or varnish, the paint will literally turn into soap, with the associated loss of adhesion, peeling and disintegration as it gets washed away by rain. Alkyd and "alkyd based polyurethane" paints won't last either because alkyd resins are basically nothing more than a "clump" of souped up vegetable oil fatty acids, and those fatty acids would turn into soap too. Polyurethane resins are really nothing more than alkyd resins with strong urethane linkages inside them, so they'd turn into soap as well. Inexpensive latex paints and primers are more alkali resistant than oil based paints, but they're not alkali resistant enough to stand up well on fresh concrete or fresh real plaster.

Quattro

Great info, thanks! I'm planning to do something similar to my exposed 18" of foundation. Although I suppose putting the 1" foam with the glued-on rocks would be a better way to go, since it will add some R value and keep my basement a degree or two warmer (maybe).

Nestor_Kelebay

Quattro:

Quite honestly, paint is probably the most poorly understood technology in the entire home center (with the sole exception of adhesives). The sum total of what most people know about paint is that you wash latex paint out of a brush or roller with water, whereas you wash oil based paint out of a brush with mineral spirits (aka: "paint thinner"). But, if you ask them what the difference is between DRY latex paint and DRY oil based paint, you get a blank stare. As a result of this prevailing knowledge vaccuum, misconceptions and misinformation pop into existance like energy in outer space and spread like wild fire. It's really a "blind leading the blind" situation.

You can learn more about latex paints than most people working in paint stores by spending a few evenings on the Paint Quality Institute web site:
Paint Information, Decorating with Paints, Paint Trends - Paint Quality Institute

The Paint Quality Institute was established by and is funded by the Rohm & Haas Company who are the largest manufacturers of polymethyl methacrylate(also known as "Plexiglas") in the world. That's because top quality latex paints use tiny particles of plexiglas as the paint binder. When you see the wording "100% Acrylic" on a can of paint, it simply means that the binder resins in the paint are Plexiglas. However, that wording really isn't a guarantee of quality because there are very many kinds of Plexiglas resins used to make paint, and they all vary in their properties, so some Plexiglas resins are better (and more expensive) than others.

The major manufacturers of alkyd and polyurethane resins (like Bayer Chemical, the Aspirin people) and the pigments used in paints (like Sun Chemical) don't acknowledge the importance of educating the public about their products, and so their web sites aren't educational at all.

DuPont is the largest manufacturer of Titanium Dioxide in the world. This is the white pigment used in paints, and plays a major role in determining the hiding ability of white and off-white paints. DuPont has an excellent section on optics on one of their web sites. Go here:
DuPont Titanium Technologies Technical Information
Then click on "eSeminars".
Then click on "Optics of TiO2".

"Hide" in paints is largely a matter of scattering light. This is why water is clear, but snow is opaque. And, to understand how to make paint hide better, you need to understand optics.


Enjoy.

Quattro

Wonderful - thanks for the links.

I was a "paint specialist" at an Ace Hardware for several years. Their "training" consisted of a few hours of video tape on how to sell and mix paint. Obviously, there was no need to inform us on the science since it would be lost on 99% of the customers.

Nestor_Kelebay

This is about the 8th or 9th online Q&A forum I've posted on since I went online about 10 years ago, but I find that people are the same everywhere.

They all want to learn the "Science of DIY" because knowing HOW something works provides "understanding", which is easy to remember because it all makes sense once you understand it. Having someone that just tells you what to do, but not why, is providing you with knowledge, but it's a chore to remember and it won't stay in your head for very long.

Whenever I come on a new board, there are always people that complain cuz my posts are too long. But, I figure that it's not just the original poster that's gonna be reading my replies, it's gonna be newbies that wanna learn to take good care of their valuable new house, and understanding that "science of DIY" provides the key. So, my posts are often criticized for being too long, but it's mostly because I take the time to explain those "whys", and that makes me popular with newbies. (They're typically most impressed when I explain how a wet t-shirt contest works. That is, why wet cotton is more transparent than dry cotton.)

Nice to meet you, and I hope this post didn't make me sound like a "know it all". I don't know it all, but I do share what I know.

Quattro

I appreciate complete answers...so no, you don't sound like a know-it-all to me!

Plus, I'm from Bemidji, so I kinda "know where you're coming from"! :-)

Bwildly

Thanks Nestor, The concrete is 40 years old and im not sure what kind of paint it was but it was previously painted 2 months before we moved in and started bubbling and peeling about 7 months after that. I have the self priming bear masonary paint from home depot to use. What do you suggest about this...It has rained a few times and splashed a little mud on my clean foundation surface. Should I just brush it off with a broom?

Nestor_Kelebay

I'd clean it off with a broom or brush since you don't want to paint over mud.

But, do you have any idea WHY the previous paint peeled off?

That suggests to me that there's moisture coming out of the concrete, and pushing the paint off. But, that moisture has to be coming from somewhere inside the house to cause the paint on the outside to peel off.

But, I can't think of anything that would cause the paint to peel over the entire surface of the foundation (except perhaps condensation forming on cold concrete indoors, and that moisture being wicked into the concrete and eventually finding it's way through the concrete to the opposite side (the exterior).

?

Nestor_Kelebay

Why don't you try this:

Go to Lee Valley or any place that sells hobbyist's supplies (cuz some people like weather stations) and buy a cheap hygrometer (measures relative humidity).

Cut a clear plastic bag open and tape the sides and bottom tightly to your foundation where the paint peeling was worst. Drop in the hygrometer (dial side out) and tape the top tightly to the foundation. The hygrometer will now record the relative humdity under the plastic.

If there is moisture coming through that concrete (which caused the previous paint to bubble and peel off), then you will soon start to see the hygrometer registering an increase in the relative humidity behind the plastic.

There are a number of other things that may have caused the old paint to bubble and peel, but the hygrometer test will ensure it's not moisture coming through the concrete, and that's important because otherwise you'd just be covering up a symptom rather than treating the cause.

What I'm thinking is that the weeping tiles around your house's foundation may be clogged up with dirt, and so they're not allowing the ground water around your house's foundation to drain away. The purpose of having weeping tiles around a house's foundation is to allow this ground water to drain away, thereby eliminating the hydrostatic pressure causing it to want to penetrate INTO your basement walls in the first place.

I don't know anything about your basement walls, but if you suspect they're wet, most plumbing companies now offer video recording of sewer pipe inspections. I can't see why they couldn't run their camera from your sump pit or catch basin to your weeping tiles and see if they're open or not.