The orange peel effect is the occurrence of irregularity of bumpy surface resembling the texture of an orange peel. 

Fig 1.

Fig.1: An illustration of orange peel on a baked industrial topcoat.

The orange peel appearance is usually due to the lack of paint spread out on the surface and is a common defect in both spray and roller applied paints. It could potentially be caused by the film's high viscosity, which prevents it from spreading. Furthermore, paint movements caused by surface tension gradient can cause the orange peel appearance as well (e.g., hills and hollows). This defect is also caused using dry spraying or fast-dissolving solvents in mixtures. If the source of the defect is the high viscosity, treatment can be achieved by lowering solids, utilizing a lower molecular weight resin, or reducing pigment loading. Spread-control additives such as silicone oils or low-energy polymers effectively prevent movements under surface tension (For details, see the discussion in the "Undercoat…" section.). The use of a co-solvent* with a high boiling point can also improve diffusion by keeping the film open in the oven for a longer time.

Fig 2.
The term co-solvent refers to solvents that leave the wet paint film later than other solvents, are good solvents for the existing polymer(s) and are used in lesser amounts in the formula than other solvents.