Shellac is an ancient , natural material; one for which I have found no substitute.  Here, the Wickipedia article, mostly accurate.

I use the white, pigmented shellac, as a base for coloring gessoed frames (an explanation of the process) prior to leafing. I mix my own using dry pigments, usually earth colors, such as Indian Red, Burnt Sienna, Yellow Ochre. Ivory Black (Bone Black) No recipes, as the color will vary with the job at hand. The various colors become an undercoat, tinting the leaf somewhat, and something to be rubbed through to for an "antique" effect. The clear and amber shades are used to seal the undercoat, and as a sealant and colorant for the leaf.

I use a "flatting" powder to soften the gloss, especially when sealing matte karet gold. It enhances the matte effect. Whiting may also be used to flatten the sealant, though it requires some care in the application. I use spray guns for gesso and shellac, except for the colored coats. Brushing gets all the recessive areas, then I spray a seal coat. When brush applying shellac, the addition of a small amount of high quality turpentine will slow the drying.

Depending on the finish for the frame, I will use , clear, blonde, amber, or button lac, or any of these with dyes added to alter the final finish. The clear and amber are available ready to apply, the others require soaking the flakes. I mostly use the Zinsser, as soaking and filtering, etc. is more expensive, and time consuming.

My choice of spray guns is the small touch up type spray gun.  This is an HVLP gun, with very little overspray, made by Binks, with a 8 oz. container. Small, easy to maneuver in and around a frame, and as opposed to the gravity type gun, with the paint container above the gun, comfortable to use. Double action, meaning more spray the more the lever is pressed, I generally set mine to a round pattern rather than a fan pattern. Fortunately, I own two of these, as Binks doesn't seem to make them any more. Very versatile gun, as I use them for thin sealants and very viscous gesso. All of my air tools are fitted with quick release connectors.