Gessoing is a messy and demanding job and once it has begun it cannot be stopped until completed or else future layers will never quite adhere properly.
Gesso, which is a mixture of hide glue and a type of chalk, is the intermediary between the wooden panel and the paint layers. It starts off as mostly glue and later more chalk is added until it produces a thick milkshake-like paint. This mixture must be put on the board while it is hot, as the glue solidifies as it cools. Once a sufficient thickness of gesso is applied, these layers have to be given a day or two to dry.
All you really need to apply the gesso mixture is a stiff paint brush, but I’ve found over the years that the best results come if you use a sprayer. By putting on many thin layers, this method allows more gesso to be applied to the panel in a safer manner. This extra depth of chalk and glue is especially important if gilding is planned.
Afterwards, the gesso is scraped and sanded producing a surface that is very, very smooth.