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Old 06-23-2007, 11:22 AM   #1
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Stucco on interior cmu/fireplace


I have a freestanding cmu fireplace in my house and I have decided to coat it with stucco or something to make it smooth (hand trowled sand finish). Since I have never worked with stucco I was hoping to get some pointers. From what I understand the cmu should be wet/moist before starting and doesn't require a latex type bonding solution. Since the fireplace dose get hot I think that I should work with only cement type products and stay clear of latex based products. What is a good product brand, or is it possible to mix my own (cement, lime, mortor etc.)? Do I just need a scratch and brown coat? How long should the scatch coat dry before applying the brown? Dose the scratch coat need to be moistened? Can you color the brown coat with cement dye? Do you use a finish trowel and a mag or a brick trowel? Any experience shared with this stuff would be great.

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Old 06-24-2007, 04:02 PM   #2
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Stucco on interior cmu/fireplace


First, let me say that you should take everything I say with a grain of salt. I never even touched a masonry product until a couple months ago. However, since that time, I've read a ton, have finished the base coat on my entire house, began my finish coat, built a concrete block/stucco mailbox, and some other masonry products. So I'm not a vet by any stretch, but have some basic experience.

CMU = concrete blocks right? If so, then yes, you'll want to make sure they're wet before applying the stucco (if not made moist, then I'd use a bonding agent. I've been using a bonding agent on things that were of questionable porousity - like the old coating on my block house. Things that were fresh concrete blocks, like my mailbox, just got misted with the hose adn then I applied stucco, no bonding agent. I should also add that I do a method of wetting that may or may not be better than simply misting it. I came up with this on my own and it could be better or worse, but I believe it's better (based on my understanding, not anything I've read). What I do is I saturate the cmu's that are waiting for stucco several times over the hour before the stucco application. I believe that this allows water to actually penetrate deeper into the cmu's, thus reducing their desire to pull water out of my stucco. Note that I only apply stucco when the cmu's are barely wet - they're just slightly damp, but I've had them soaked so I know (or assume) that there's much more moisture behind their surface.)

Some more pointers

if you're doing this inside, and never worked with stucco before, be ready for the mess! Have paper towels handy, and I'd recommend using painters' tape and tarps, because if you're like me you'll be making a mess.

Consistency of the stucco is important. Too wet of a mix will make it more workable (to an extent - if it's all soupy then it's kind of a pain to work with too), but the more water you mix it up with, the weaker your final product will be. I can't explain slump of the mix properly, I could only show you in real life, but this is kind of important. Too dry, and it's a pain to work with, too wet and it'll be weaker, waaay too wet and it's a pain to work with and it'll be weaker.

Curing - curing is very important to your finished product. You don't want it to dry out too fast. Any wall that I do is kept moist by 1 of 2 methods, depending upon location (sometimes 1 method's more appropriate given what / where the stucco is). One is to tarp it, just cover it so that it can't lose it's moisture easily. The other woudl be to continually give it a light misting of water. Proper curing is vital.

How long between your coats? I've read all over the place, from 24 hours to a week. I typically won't put another coat over stucco until it's been at least 3 or 4 days, and I keep it tightly tarped (or constantly misted - my work is outside though, so inside the mistings wouldn't need to be as frequent, I need to mist frequently on non-tarped walls because the FL sun dries them out waaay fast). So I won't even consider putting up another coat for at least several days, since during those days I'm still curing my current coating. Oh and when I put up a new coat of stucco, the previous coat of stucco gets the same pre-wetting that the original CMU's got - heavy wetting for an hour or so beforehand, to the point I feel I've got moisture to a good depth inside the coating, then wait til it's only slightly moist/damp before applying.

You'd want only 2 coats on your application (assuming cmu's = concrete blocks). I typically see 3 coat applications recommended when using lathe (typically on non-masonry surfaces), and 2 coats when applying to a masonry surface.

Yes, you can add color to the mix directly, or paint it. Keep in mind that it's recommended to wait for the stucco to cure a bit before painting. I'm not painting mine - I used generic cheap gray stucco as my base coat, and bought expensive white stucco for my finish coat. I'm adding (sakrete brand I believe) tan color pigment powder to my white finish coats, at a ratio of 1 box of color to 3 bags (60lbs bags) of white stucco, giving a light tan color.

As far as trowels, I have some ghetto beaten down trowel that I got for free that I use for everything - applying base coats, finish coats, smoothing, etc. I know there's many options with trowels, but I've never had a problem with the garbage trowel that I have, no matter what I tried doing with it.


When you do your base coat, you'll want to score/rake it. I use a nail, and just draw a bunch of horizontal lines into it. These lines act as a key for the next stucco coat (and allow a spot for moisture to hang out when you mist it).

Another note about curing - my gray base coats go up, and if the sun's beating down hard, you see some areas lighten up really quickly. So keep an eye on your stucco, if you're going slowly you may want to mist the first spots you did before you're done wiht your project (that happens with my walls - the sun will be drying the spot I started on before I even finish the wall!)

Let's see, what else... I'd recommend a larger trowel and a smaller one (I'm not even sure if my small one is called a trowel, it's more like a large chisel/spackle piece). I use the small piece to do edges and small details, spots that would be hard to maneuver with the large trowel.
Make sure you have enough latex gloves on hand! Stucco dries your hands out real bad if you're not wearing gloves.

When applying your stucco, begin at the bottom and work upwards. Make sure to use proper pressure to push it into the substrate as you move the trowel upwards.

I believe my brand was amerimix stucco (my base coat). Can't remember the brand of my white stucco. On such a small project, I don't see much reason for learning how to mix it up yourself, but maybe you do? I just went with premades, worked well for me.

Good luck!

(Oh! 1 last thing - for a really smooth finish, you may need to mess around with troweling for practice first. I know I wouldn't have trouble doing a smooth finish now, but when I was doing my first wall, no way I could've done a smooth finished wall.

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