Gesso is the oldest type of primer; it essentially consists of a white pigment and a water-based binder, and is used to prepare a surface for subsequent layers of oil paint. It does this by sealing the surface (much like glue size), as well as by eliminating the texture of the underlying material by providing a soft, sandable layer on top of it. However, like many other coatings, gesso has been made unnecessarily complex; modern formulations contain sophisticated polymer emulsions, while many traditional recipes are misinterpreted as requiring specific (expensive) ingredients and complex processing steps. None of this is necessary, however; the word "gesso" is simply a translation of "gypsum", and as such, I have developed a method for priming surfaces which uses nothing more than plaster of paris and gelatine.
My first discovery was that plaster of paris is suitable on its own as a water-based putty. There are a number of commercial water-based putties available, but after researching their ingredients I found that most are simply plaster with added pigments and fillers, and as a result they perform no better than plaster alone. Therefore, the first step in this method is to use plaster (mixed with water to form a thick paste) to fill any major defects in the underlying surface. Once this has hardened, it can be sanded down as needed; these steps can be seen at the left and middle of the top of the page.
From here the actual gesso can be prepared, and this step takes no more than a few minutes from start to finish. I simply mix equal weights of plaster and water to form a thin slurry, then add powdered gelatine at five percent of the weight of the plaster. This rapidly absorbs water and thickens the mixture, at which point it can be warmed gently until the gelatine melts, and the gesso is ready for use. By mixing the plaster and water first, the plaster becomes completely dispersed (with no need for grinding or straining), and in combination the plaster and gelatine have an interesting chemical partnership: the addition of gelatine delays the setting of the plaster, while the slight solubility and acidity of the plaster has the same effect on the gelatine, allowing it to be applied for some time at room temperature. The gesso brushes easily, dries within an hour, and a single coat is adequate for priming, although applying three coats allows for an opaque and smooth surface when sanded. These steps can be seen at the top right of the page.
In terms of functionality, this gesso performs far better than I expected. Although I initially intended it strictly for priming wood, it appears to adhere strongly to most surfaces, including glass and rubber, and on flexible substrates a thin layer will bend without cracking. I found no major issues during application, with the only defect being the slight tendency of the gesso to contain bubbles of air; however, I found that adding a drop of clove oil to a batch of gesso rapidly eliminates any bubbles, by increasing the surface tension and acting as a kind of anti-detergent. This does reduce the brushability of the gesso somewhat, but I found no issues when applying it in thin layers, and the pleasant smell is an added bonus. As a primer the gesso does its job well, allowing paint to be applied without losing its oil to the underlying material. It also appears to have some covering power of its own, as a layer of white paint applied over gesso is totally opaque, whereas the same paint applied over bare wood is partially transparent. Overall, the only true limitation I found is that a batch of gesso must be used in one day, with any remainder being discarded; if left overnight, it will solidify completely and become difficult (but not impossible) to remove from its container.
While this method can obviously be used to prepare panels for artwork, I believe this may also be useful for functional objects such as furniture or machinery, so long as the object in question is intended for indoor use. Having the ability to smooth and flatten the surfaces of damaged or re-used lumber seems like a versatile addition to my workshop methods, and I intend to refine these techniques further as I use them in future projects.