Ventilation is very important during the plastering process to achieve a smooth finish. Proper ventilation helps maintain a consistent temperature and humidity level, which can significantly impact the plaster's drying and curing time. The plaster can become dry and moist with adequate ventilation, leading to cracking, shrinking, or uneven texture.  Also, poor ventilation can cause moisture to build up on the plaster's surface, creating watermarks and other imperfections. By ensuring good ventilation, you can help ensure the plaster dries and cures evenly, resulting in a smooth and uniform finish.