Water tank rendering/waterproofing?

We have seen many of these on YouTube tours of central Portugal farms. We never caught exactly what materials they were using. They are typically water catch basins for irrigation but double as a plunge pool from what we saw. We saw this link with what we can tell could be a nice way to go.

I have no experience with that but will had my two cents stating that almost certainly the temperature will impact the application, any product that you use for this will have an ideal range of temperatures to be applied, and if nothing else because when applying paints or waterproofing materials the higher the temperature the faster they dry so the application is harder, the same with the temperature of the material that is being treated.

I’ve waterproofed my tile roof twice and have to do it in the morning when the sun is barely risen and as soon as it its the roof I have to stop, because the tiles warm up and it makes the product hard to distribute, also the product dries in the bucket, it forms a film above the material which also difficult the application and leads to bad results.

I would say the best time to apply something like this would be in early morning because the walls have cooled down from the night, just make sure they are dry without moisture.

As a side note, if someone is thinking of waterproofing rooftops chose it in white, I did the mistake of choosing grey, so it did not get so dirty and I’m paying the price, the house is much warmer this year, I will have to go back to white. But as a bonus I get warm water from the sun, my water pipes run in the ceiling and my cold tap water is at a balmy 36ºC.

Many thanks Jones' and Sonia.  We are in no hurry to waterproof the tank so can continue exploring the options and await the cooler months - as long as it is done before the first heavy dew/rain.