A halo refers to visible applicator marks that don't match the existing painted surface. These can result from brushing the edges and around light fixtures after the ceiling has been rolled, from using a brush or roller to touch-up a sprayed ceiling, or from applying a second coat of spray paint mixed to a different consistency than the first coat. Poor quality paints make it more difficult to seamlessly touch up a surface. Depending on the cause, eliminating a halo can be as easy as using the correct applicator, or in worst case scenarios, recoating the entire ceiling.
Wait at least 24 hours after you've done the touch-up before trying to eliminate halo marks. Sometimes paint needs some time to dry and blend in, especially if the air is cool and humid, which can significantly retard drying time.
Use the same method of application for the paint touch-up. In other words, if the ceiling was rolled, use a roller to touch up; if the ceiling was sprayed, touch it up with a paint sprayer.
Run your hand over the halo and sand it down with a sanding sponge or 120-grit sandpaper if it feels rough. Some halo marks are caused by sprayed paint being applied too dry, which will look (and feel) like small raised speckles of paint, according to a Painting and Decorating Contractors of America fact sheet.
Thin the paint with either water or paint conditioner. Use up to a pint of water or conditioner per gallon of paint. Apply the thinned touch-up paint well beyond the margins of the halo mark to blend the new paint with the old.
Cut in (paint all the edges with a brush) before rolling a ceiling. Cutting in after the ceiling has been rolled often leaves visible brush marks, which will be visible as a halo around ceiling fixtures.