CAUSES: Inadequate sealing of surfaces with varying porosity may, after painting, cause a non-uniform sheen. If shiny are noticeable, this problem is sometimes referred to as “flashing”. Uneven application can result in varying film thickness and uneven gloss. Fog-strike, or moisture on the paint film during drying, flattens high-gloss finish.
Temperature variations during drying also affect gloss. Areas surrounding nail heads and wall studs tend to retain glass because metal conducts heat and speeds up drying time; also metal are non-porous which improves gloss. In addition, painting over a paint film that is not thoroughly dry or that is too soft can cause uneven gloss.
SOLUTIONS: If the surfaces is extremely porous, a prime coat is necessary; otherwise another finish coat, spread uniformly, might correct the problem. Some unevenness can be expected on rough surfaces, but additional coats will give better uniformity.
If moisture contact has caused flattening, or if temperature variation has occurred, you will have to apply another coat of paint when moisture is not present and/or when changed in temperature are less likely.
If the undercoat was not dry, allow the flattened paint to dry hard and apply another finish coat. To make sure the paint is dry, scratch the surface with thumbnail.