Their principal application is to modify nitrocellulose lacquers and lacquer sealers, in order to impart gloss, hardness, and easy sanding. It has been said that the commercial production of drying alkyds modified with natural resins was as important a development in the surface coating industry as the discovery of the alkyds themselves. _Drying alkyd resins modified with other synthetic resins._—Drying alkyd resins may be modified with tar-acid formaldehyde resins, tar-acid furfural resins, urea-formaldehyde resins, petroleum resins, and the coumarone and indene resins. Modification with tar-acid resins gives a quicker setting, harder drying finish with a higher gloss. Alkyd resins so modified are adapted to both air-drying and baked undercoats and finishes; they have good durability and adhesion and good resistance to grease, oils, alcohol and abrasion. For some uses the tar-acid resin modification gives better qualities than either component possesses alone, but in light colored finishes it has a tendency to cause the finish to yellow. Coatings made of drying alkyd resins modified with tar acid resins are widely used on automobile chassis, fenders, and bodies, machinery coatings, steel fixtures and toys; they are especially suitable for primers, undercoats, and finishes on metal. Modification with urea resins produces baked-finish coatings. As much as 40 percent of the urea resin is incorporated. It makes possible coatings with a full range of permanent colors and improves their hardness and mar-proofness, whereas without the ureas the combination of color range with hardness had been difficult to obtain. The urea resin modified alkyds find use on metal surfaces of articles which must stand rough handling, such as toys, furniture, and motors. Modification with petroleum resins produces air-dried finishes. For industrial use on metal they give coatings with better adhesion, dispersion of pigments, and resistance to acids, alkalies, and moisture at a lower cost than is obtained by ester gum or tar-acid resin modification. The petroleum resin modification minimizes skinning and improves the luster and the flow. _Drying alkyd resins modified with other synthetic resins and oil extended._—Excellent water resistance and versatility are the characteristics of finishes made of alkyd resins modified with other synthetic resins (usually tar-acid) and oil extended. The incorporation of drying oils gives a low cost finish with better compatibility and brushing and with the combined properties of a quick-setting varnish and an alkyd resin. Although not so durable or quick setting as the unmodified finishes, they have better water resistance. These finishes may be brushed or sprayed, air-dried or baked. They have wide industrial and architectural uses.