The final brilliance of a gilded surface depends entirely on the quality of its preparation. For oil gilding on porous materials like wood or plaster, the first step is to create a non-porous barrier. This is typically done by applying a sealant. Without it, the size, or adhesive, would soak into the substrate, leading to a patchy, dull finish.

Shellac, a natural resin, is the traditional and most common sealant for this purpose. It's thinned with denatured alcohol and applied in one or two thin coats. This effectively isolates the substrate, ensuring the size rests evenly on the surface, ready to receive the gold leaf.

Water gilding, which allows the gold to be burnished to a mirror-like shine, requires a more intensive preparation. The goal is to build up a perfectly smooth, slightly flexible ground using gesso. This isn't the acrylic gesso found in art supply stores, but a traditional mixture of an animal-skin glue, usually rabbit skin glue, and a whitening agent like calcium carbonate (chalk) or calcium sulfate (gypsum).

This mixture is heated gently in a double boiler to keep it warm and liquid. It's then applied in multiple thin layers, often between eight and twelve, with each layer allowed to dry before the next is brushed on. Each new coat is applied in a direction perpendicular to the last to ensure an even buildup.

Once the final layer of gesso has cured completely, typically after 24 hours, the surface must be sanded to absolute smoothness. This process, often done with a little water as a lubricant, is what creates the flawless foundation needed for a high-luster finish. The sandpaper grit progression is key.

You might start with a 220-grit paper to level any major imperfections, then move methodically through finer grits: 320, 400, and finally 600 or even higher. The goal is to remove the scratches from the previous grit, resulting in a surface that feels as smooth as polished stone.

After sanding, the final preparatory layer for water gilding is applied: bole clay. Bole is a fine, colored clay mixed with a small amount of the same rabbit skin glue solution. It is brushed onto the gesso in two or three thin layers. The bole provides a slight cushion for the gold leaf, making it easier to burnish. It also provides a rich undertone that can subtly affect the final color of the gold.

Metal presents a different challenge. It's non-porous, so sealing isn't the primary concern. Instead, the focus is on cleanliness and adhesion. Any oil, grease, or residue on the metal will prevent the size from sticking. Therefore, the surface must be thoroughly degreased using a solvent like denatured alcohol or acetone.

After degreasing, a high-quality metal primer is applied. This primer serves two purposes: it protects the metal from oxidation and creates a uniform surface with enough 'tooth' for the gilding size to adhere to properly. Once the primer is dry, the surface is ready for the application of oil-based size.

With the substrate perfectly prepared, it's now a stable and receptive canvas for the adhesive and leaf to come.