Which statement about the mixing of grout is true?

Mixing grout properly is essential to achieve the desired consistency and performance in construction projects. The statement indicating that it must be mixed until no lumps remain is correct because proper mixing ensures a uniform consistency that maximizes the grout's adhesion properties and minimizes the risk of points of weakness. When grout is mixed thoroughly, it enables even distribution of water and additives, leading to a robust final product that can effectively fill gaps and spaces in tiling work or masonry.

In contrast, the other statements do not accurately reflect best practices for mixing grout. For instance, mixing dry grout fails to incorporate the water needed to activate the product fully. While grout can be mixed by hand, it is not limited to that method; using mechanical mixers may achieve more consistent results. Additionally, while exposure to rain is certainly not ideal for grout that is not yet set, it is inaccurate to say it should never be exposed at any time, as conditions vary depending on the project's requirements and the type of grout used. Hence, the correct choice emphasizes the importance of mixing until a smooth, lump-free composition is achieved for optimal application results.