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This is going to largely depend on the paint itself as the thickness of the coating is variable depending on the substrate absorbency and the specific formulation of the paint, you then have specific variables for paint thicknesses.So for example a mineral paint will have a defined thickness, 50 to 80 µm (micrometres) based on 2 coats, but the second coat doesn’t add to the first coats thickness in a typical manner, it doesn’t double in thickness due to the paint absorbing and binding to the previous layer of paint.
Ultimately with any paint, the more coats you apply, the thicker it becomes. A thicker layer will become less vapour permeable.
However paints then degrade over time due to things such as weathering and pollution, which again impacts the vapour permeability, this can be positively or negatively by opening or closing the pore structure.
For example with a mineral paint due to their longevity, the number of required coats/recoating is significantly reduced and as such you will likely never realistically reach a thickness that would prevent vapour permeability. With a pure limewash, you are only adding lime, so you don’t have other additions which will impeded or restrict vapour permeability, so the number of coats required to restrict moisture movement will likely never be realistically reached, you also have the poor durability of limewash, which weathers away layers over time and reduces the thickness of the coating.
For other paints, it all depends on the specific ingredients used.
Please let us know if we can be of further assistance.