Why do both spraying and rolling sometimes produce a mottled 'orange peel' effect?

When spraying, insufficient thinning can create this 'dimple' effect on a paint coating. The same is true when using a roller, or it could be the wrong type of roller for the product used. This is normal for such methods of application. To minimise roller mottle, use the shortest pile roller suitable for the finish In any case, to correct the fault rub down with wet & dry abrasion, using water or a suitable solvent, then rinse down and allow to dry thoroughly prior to applying a new paint coating.