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?
Best Answer
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).