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I am building with AAC blocks in a tropical country. The usual practice here is to apply a home-made plaster/stucco (cement/sand mixture) to the wall (which here is more typically mortar). However, the recommendations for AAC suggest that a polymer-modified plaster is more appropriate, with the warning that the stucco/plaster is liable to crack.

Any thoughts on this matter?

I have purchased a pre-mixed skim coat plaster product, which I intend to apply to the home-made plaster. But I am not sure if I am setting myself up for trouble by using the normal plaster rather than the expensive pre-mixed product?

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Congrats on your choice of materials, FYI. AAC is great stuff, totally appropriate for your climate.

Applying stucco over AAC isn't a fundamentally different process than applying it over mortared block or brick, so if most stuccoed masonry buildings in your area aren't full of cracks, you should feel more at ease. The only real difference that AAC may present a smoother surface, which could lead to worse bonding compared to a rougher brick or block. That's probably why the instructions call for a polymer-modified plaster; the polymer additives make it stickier, to promote superior bonding. The traditional way to promote bonding is with metal lath. However I honestly wouldn't recommend this in your climate since moisture will eventually rust it and the stucco will come off (rusting metal expands, cracking the stucco).

You have three options: obtain a polymer additive and instruct the stucco crew to add it to their stucco mix; hire someone to mechanically roughen up the outer wall surface before the stucco crew arrives; or let them do their normal thing and hope for the best (it will probably be fine).

  • thanks. Do you have any thoughts about bare AAC walls? I have neighbours on my left and right. The left side has just a couple of inches space to the next building and it's not possible to plaster it, although I could apply a resin coating to the blocks before placing them (I am not sure if this is a good idea). On the other side there's about a foot of space, so it's possible to plaster the wall I suppose, but not really to paint. I am not sure what will happen to the bare blocks if exposed to tropical weather. And will I be ok to paint the blocks on top of the skim-coat without primer? – thelawnet May 14 '16 at 18:08
  • Bare AAC should be just fine. There's no concerns about freeze-thaw damage where you live, so just protect them from water with roof overhangs. Any coating you apply should be vapor-permeable to allow any moisture to escape. If you paint with regular latex paint, it will bubble and peel as moisture in the wall tries to migrate outwards through it. A good vapor-permeable sealer is sodium silicate, which waterproofs but permits water vapor to escape. This is also a great additive to put in a stucco or plaster mix! – iLikeDirt May 14 '16 at 18:53
  • I would add coloring to your mortar so you don't need to paint. My grandmother's house was stucco that had the pre colored mix and when she passed we pressure washed the outside and it looked like it had just been painted. The house was 60 or 70 years old and had never been painted. I have used the coloring several times the hardest part is keeping the amount you add to each batch on the skim coat consistent so there is no real color variation. – Ed Beal Oct 22 '18 at 21:55
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just get your stucco contractor to apply a fiberglass mesh lath. this is SOP for all EIFS stucco system, and can easily and cheaply be added to your stucco procedure.

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