Watercolorists traditionally paint whites by leaving the white of the paper. To do so, these areas need to be protected from being painted inadvertently. This means you must plan ahead for where those areas are going to be and take care not to paint over them by mistake. An easier way to handle this is to mask them with pieces of tape or masking fluids. Masking fluid (sometimes referred to as liquid frisket) can be applied with an old, worn-out brush (never use a quality, or new brush) to those areas that are to be protected then allowed to dry. Once the surrounding area is finished being painted, the masking fluid can be removed to once again expose the white of the paper underneath. To remove the mask, rub it gently with a clean dry finger or rubber cement pickup.