Poor sheen uniformity leads to shiny spots or dull spots (also known as "flashing") on a painted surface.
What Causes It?
Unevenly spreading the paint as you apply it or failing to maintain a wet edge, which can lead to lapping, are two common causes of poor sheen uniformity.
This condition can also develop if a porous surface or a surface with variable porosity was not properly primed before the paint was applied.
New substrates should be primed and sealed before applying the top coat to ensure a uniformly porous surface. Without the use of a primer or sealer, a second coat of paint will more likely be needed.
Make sure to apply paint from "wet to dry" to prevent lapping. Often, applying an additional coat will even out sheen irregularities.
Note: images provided by The Rohm & Hass Paint Quality Institute.