Venetian plaster is a wall finishing technique that is made by applying thin layers of plaster to a wall with a trowel, then burnishing the topcoat. This produces a polished marble-like finish with an illusion of depth and texture. The basic plaster mix is a combination of gypsum, cement, and sand, often with ground marble included. The plaster surface is often protected with a coat of wax. You should always use the sealer or wax recommended by the manufacturer of the Venetian plaster to make sure it will adhere properly
Allow the Plaster to Dry
Make sure the Venetian plaster is totally dry before applying wax. Leave it for a full 48 hours to be sure.
Apply the Wax
Lightly burnish the surface with a trowel then load the trowel or spatula with wax. Using the trowel, apply a thin layer of wax over the finished plaster. Spread a short stroke of wax with the troweled surface almost parallel to the surface, without the blade touching the wall. Leave a very thin film. Allow it to dry then trowel over the second layer of wax. Apply over the entire surface in overlapping patches. Allow it time for proper drying
Burnish the Finish
You can now burnish the wax coat with your trowel. Rub the trowel over the wax with the full flat edge of the trowel against the wall. Make sure you use the face of the trowel as the edge can cut the wall if not used carefully.
Get a high gloss by rubbing in circles. You can use an electric polisher if you are very careful with it.
Another Way to Wax
You can apply the wax with a sponge or cotton cloth, applying as many coats as needed until the plaster stops absorbing the wax. Apply the wax by hand but you can use an electric polisher. Be careful not to make streaks. Work section by section. Re-wax any areas where streaks appear immediately.
Polish the surface with a dry non-woven cloth and then buff it with another non-woven cloth or electric polisher.
Using a roller or garden sprayer, apply a solvent or water-based sealant to the surface and allow it to dry completely. Apply a second coat and wipe off after five minutes with a sponge or rag. You must wipe off the excess sealant or you will end up with a white residue on the surface. Test on a small area first.
If you are waxing with iridescent powders, liquefy the wax by stirring vigorously and pour in the amount of iridescent powder needed. Mix well. The wax will return to a normal consistency when you store it in a cool place, even a refrigerator.
You can add a great aged quality to the finish by color-washing. You can make a color wash using water-based glazing liquid or acrylic colorant. Use a paper towel or rag to apply the color in sections and wipe off. If the result is patchy (check a small section first), apply a water-based penetrating sealant before glazing.
Both plastering and waxing are very labor-intensive so you should get a helper to make the job easier.