The most common cause of hairline ceiling cracks is plaster shrinking during the drying process. However, changes in temperature and humidity, or roofing timbers shrinking and expanding, could be contributing factors. Although hairline cracks don’t affect structural integrity, they are noticeable under certain lighting conditions. Wait three or four weeks after the first cracks appear for the plaster to dry thoroughly before taking action.
Scrape off any high spots, loose paint or debris with a putty knife. Sand the scraped area smooth with 120-grit sandpaper and a sanding block.
Prime the surface with a semi-gloss acrylic paint using a long-handled paint roller. This will create a good bond for the plasterboard compound and give you extra working time with.
Mix plasterboard compound powder with water in an empty paint bucket. Mix until the compound reaches a fairly thick creamy texture. Use a low speed electric drill and a mixing paddle for the best results.
Stand on a stepladder and apply a thin coat of compound to any wide cracks with a paint brush. Cover the cracks with a strip of fibreglass drywall tape and smooth down; the compound will cover all hairline cracks without the need for tape.
Paint a metre wide strip across the width of the ceiling with plasterboard compound, using a fresh long-handled paint roller. Apply the compound in a fore and aft motion in a paint-like fashion.
Dip a long-handled flexible trowel into a bucket of clean water. Simply wipe the compound smooth by applying moderate pressure and wiping over the compound in the same direction as the roller. Rinse the trowel in clean water after smoothing down the strip.
Continue in this manner with slightly overlapping metre-wide strips until the entire ceiling is covered and perfectly smooth. Do not forget to wet the trowel after each pass. Allow the surface time to dry. Use a stepladder and sand down any faint trowel marks and imperfections you find.
Apply a second coat of compound over any low spots you detect. Remove the long handle and use the trowel to wipe the area smooth. Allow adequate drying time and paint the ceiling with the colour of your choice.
Wear safety glasses when sanding the ceiling.