Manufacturers press bits of wood, saw dust and adhesive to form particleboard. Leaving a window open during rain or periods of high humidity can cause particleboard to swell and bubble. Successful repairs depend on the severity of the bubbles in the particleboard. Repairing a windowsill with minor bubbling is possible, but a severely bubbled or warped windowsill requires replacement. Some manufacturers make a moisture-resistant particleboard that endures light water contact. Swelling that occurs between the paint and particleboard rather than to the particleboard itself, making this repair project easier and more successful than repairing bubbling particleboard.
Cut an X through the bubble with a utility knife to determine if the swelling is between the paint and particleboard or if the particleboard has bubbles. If cutting through the bubbles is easy and does not touch the particleboard, bubbles are between the paint and particleboard.
Set up a fan and direct the airflow toward the bubbles if the particleboard feels wet to the touch. Let the airflow stream toward the bubbles for several hours until the board feels dry to the touch. Drying times vary based on the temperature and humidity.
Sand the particleboard surface lightly with 80-grit sandpaper to remove the paint from the bubbles and around the bubbles.
Wrap 120-grit sandpaper around a sanding block. Sand the windowsill to remove any remaining paint and smooth the particleboard bubbles flush with the rest of the windowsill.
Suck up paint and dust with a shop vacuum.
Paint a coat of all-in-one latex primer and paint with a paintbrush. Let the first coat dry for 2 to 4 hours. Paint a second coat of all-in-one latex primer and paint.
Wear a dust mask, gloves and eye protection when sanding particleboard. Keep a particleboard windowsill protected with paint at all times. Replace severely bubbled particleboard windowsill with a more durable windowsill.
Do not sand wet particleboard, as sanding may cause large pieces to dislodge from the windowsill.