For a flanged connection to be watertight, the faces should be what?

For a flanged connection to be watertight, the faces should be machined. Machining the flanged faces ensures that they are flat, smooth, and evenly aligned, which is crucial for achieving a proper seal when the flanges are bolted together. Any imperfections, such as nicks or uneven surfaces, can lead to gaps that allow fluid to leak through, compromising the integrity of the connection.

The machining process removes any irregularities and creates a surface that tightly fits against a gasket or another flange face. This solid, even contact helps to prevent leaks and maintain pressure in the system.

Other options like coating with oil, painting, or polishing do not provide the necessary functional attributes required for a watertight seal. Coatings can create a barrier rather than a tight fit, while painting may lead to uneven surfaces that could allow leakage. Polishing can sometimes provide a smooth surface, but without a precise machined flatness, it is not sufficient for ensuring that the flanged connection is watertight.