Underatomized- Not dispersed or broken up fine enough.

Undercoat- For unpainted surfaces, the coat between the primer and the topcoat. For previously coated surfaces, the undercoat is applied directly to the old paint.

Uniformity- Not varying in gloss, sheen, color, hiding, or other property. 

Urethane- A product resulting in a tough, chemical-resistant finish. Urethane requires mineral spirits for thinning and cleaning up.