Solid materials such as aerated concrete wall-building materials must have an effective "outside" weather protection.
An insufficient protection would be just a painting. In rain or thermal influences painting is not in a position to withstand the tension, or the penetration of the wall for long periods to prevent it. An outer plaster must be elastic, waterproof and weatherproof. Furthermore, YTONG suggests dry-mortars from quartz sand because of they cooperate better with YTONG blocks.