The L3954F engobe is tuned to have the same degree of vitrification as this porcelain (P300). I made a pint of a brushing version of it by mixing a 500-gram batch with 75g of Laguna Gum Solution and 280g water (it does not require VeeGum because it already contains a high percentage of plastic clay). Mixing in a kitchen blender gets all the lumps out. It paints beautifully onto leather-hard ware in sufficient thickness that one coat covers (this enables presenting this normally white burning body as a black porcelain to match the glaze). I make my own labels for single-jar brushing glaze and underglaze tests. They clearly show the code number, we assign these in our insight-live account for the project. Subsequently, the piece was bisque-fired, black-glazed, and fired at cone 6. The band painted on the base, which I did as a fix-up for a few tiny white bare spots, demonstrates something unexpected about this homemade engobe: It paints well on a vitrified surface, dries there without cracking and even fires there without flaking off.
Engobes are high-clay slurries that are applied to leather hard or dry ceramics. They fire opaque and are used for functional or decorative purposes. They are formulated to match the firing shrinkage and thermal expansion of the body.