Luster
Eggshell paint has a slight luster. It is much less shiny than gloss paint. Flat paint contains no luster and does not reflect light in the same way.
Stain Resistance
Eggshell paint resists stains better than flat. Eggshell finished may be wiped with a wet rag to remove the stain, but flat surfaces just absorb the water.
Concealment
Flat paint is best for concealing dents and other imperfections in the wall. Eggshell shows some imperfections due to the reflection of the light.
Texture
Flat paint produces a rough texture that scratches easily. Eggshell is smoother and has a slicker texture.
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Finish
Flat paint produces an even and uniform finish. It is a good choice for drywall. Eggshell does not appear as smooth and uneven paint jobs are more obvious.