Blocking is when two painted surfaces stick to one another when they’re pressed together (I.e. a window sticking to a sill). When the surfaces are stuck together and then peeled apart, there is usually peeling or flaking. Blocking happens when a painted surface is not given enough time to dry before coming into contact with another painted surface.
To avoid blocking, make sure to start with high quality paint and primer. Ensure your paint surfaces have fully dried before they come into contact with any other surface. Don’t apply too much paint to an area, and make sure that your first layer of paint has dried before painting your second coat. Also, ensure doors and windows, and any other location where blocking is likely to occur, are in top working order before starting to paint.
If blocking has already occurred, scrape and sand the area to remove lose paint and clean off any dust. Prime the area with a quality primer before repainting.