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Commercial render/stucco mixes are designed to be partly waterproof. This is to reduce transmission of moisture while still allowing them to be partly breathable.The evidence of this becomes apparent when adding water to them it is difficult to get the water to penetrate.Most commercial renders are cement based and come dry in bags. Some will have “poly modified” marked on them, but they are still a cement based render. You can use a 100% acrylic render, these come wet in a bucket, but are a much more expensive option. The layer should be reinforced and the most common method is to use chickenwire. However because it requires applying over a compound curve and the mesh needs to be in the middle of the layer, this operation takes ages. My solution is to use random Alkaline Resistant AR glass fibres. They are very easy to use, saves heaps of time, but they are expensive. Seek them out from the concrete countertop manufacturers.
Renders are usually applied in two layers, but to save time I do it in one layer around 15mm thick. The bagged renders will also recommend a max thickness of around 10mm, but because of the fibre addition you can go thicker. Hydrated lime in the mix increases flexibility, an advantage when the layer is exposed to thermal expansion that won’t be even over its whole surface. Lime also has crack healing properties which is an additional advantage. Unfortunately it has been either removed or reduced in proportion in most commercial renders because of the safety issue, make sure you wear rubber gloves.
A good brew is 4:1:1 fine sand, Portland cement, hydrated lime. This will not have any waterproofing however. That can be done by applying an impervious layer over the render later.
Regarding wetting the vermicrete before application, I don’t bother because it will be absorbed almost instantly. As it is also going to want to draw moisture from the render layer. My solution is to trowel the surface until all tooling marks are gone, then wet sponge the surface, then wrap the whole oven in clingwrap sealing the whole thing. This seals the moisture in the outer layer. The evidence of a proliferation of moisture beads under the cling wrap that remain until it is removed, indicates a good moisture level is maintained. As the strength of anything made with Portland cement will be enhanced by extended damp curing, I give it a week before removal.
I stress that this is just my approach, there are many solutions.
Renders are usually applied in two layers, but to save time I do it in one layer around 15mm thick. The bagged renders will also recommend a max thickness of around 10mm, but because of the fibre addition you can go thicker. Hydrated lime in the mix increases flexibility, an advantage when the layer is exposed to thermal expansion that won’t be even over its whole surface. Lime also has crack healing properties which is an additional advantage. Unfortunately it has been either removed or reduced in proportion in most commercial renders because of the safety issue, make sure you wear rubber gloves.
A good brew is 4:1:1 fine sand, Portland cement, hydrated lime. This will not have any waterproofing however. That can be done by applying an impervious layer over the render later.
Regarding wetting the vermicrete before application, I don’t bother because it will be absorbed almost instantly. As it is also going to want to draw moisture from the render layer. My solution is to trowel the surface until all tooling marks are gone, then wet sponge the surface, then wrap the whole oven in clingwrap sealing the whole thing. This seals the moisture in the outer layer. The evidence of a proliferation of moisture beads under the cling wrap that remain until it is removed, indicates a good moisture level is maintained. As the strength of anything made with Portland cement will be enhanced by extended damp curing, I give it a week before removal.
I stress that this is just my approach, there are many solutions.
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