Uneven Gloss (Flashing)

Uneven appearance of a coating's gloss, sheen or luster due to deterioration of the paint film.

Use of a gloss alkyd/oil-based paint or solvent-based epoxy in areas of direct sunlight.
Heavy dew, moisture, or condensation getting onto a coating (e.g. primer, paint or stain) while drying.
Temperature fluctuations during drying.
Coating over a porous surface.
Paint applied at uneven film thickness (i.e. lapping).
Insufficient film build (e.g. coating applied too thin).
Use of an interior paint outdoors.

Exterior alkyd/oil-based paints and solvent-based epoxies will chalk, giving the appearance of loss of gloss failure. Washing the chalk off should return most of the original finish, but it will chalk again. If this is unsatisfactory, recoat with an acrylic following all label directions.
Bare substrates (e.g. the surface to be coated) should be primed/sealed before applying the topcoat to ensure a uniform surface.
Spot prime any patched areas to try to level off the porosity of the surface.
Often, applying an additional coat will even out sheen irregularities.
Thinning any touch-up coating may help it blend in better.
Follow label and data page directions for the proper environmental condition for application.
Follow label and data page directions for the appropriate film build and applicator.
Follow label and data page directions for product use.