Electronic devices may have housings in which components are mounted. Some of the components may be sensitive to moisture. Other components may be insensitive to moisture and may form openings in a device housing that allow moisture to escape from within the housing. Components may be mounted on substrates such as printed circuit board substrates. Moisture repelling layers and moisture attracting layers may be patterned to form channels and other structures that guide moisture away from sensitive components towards insensitive components. Moisture repelling and attracting layers may also be used to limit the lateral spread of a conformal coating layer when coating components.