Resins used mostly in trim paints, inside and out, although some medium duty equipment and marine enamels employ these resins as binders. Most often alkyd resins are found in vehicles employing aliphatic hydrocarbons (mineral spirits or other refined petroleum distillate) as thinner. Alkyds offer good leveling properties and cure to a relatively durable film, but tend to yellow interior and embrittle with age. Color and gloss exterior is only fair, and alkyds are highly prone to failure exterior on surfaces containing even moderate levels of moisture. Chemically, alkyds are synthetic resins formed by the condensation of polyhydric alcohols with polybasic acids. They may be regarded as complex esters. The most common polyhydric alcohol used is glycerol, and the most common polybasic acid is phthalic anhydride. Modified alkyds are those in which the polybasic acid is substituted in part by a monobasic acid, of which the vegetable oil fatty acids are typical.