Lime Based Venetian Plaster Application Tips

Matching Plaster

• Venetian plaster in historic buildings often varies significantly from one structure to another. It may be applied in the traditional seven coats, each one thinner than the last, or the plaster may have been applied in a few thicker coats. Some plasters are very flexible and contain large amounts of animal hair or cellulose fiber, while others are very smooth. The Building Conservation Directory points out that while it can be tempting to assume there's just one right way to plaster, better results come from matching the new plaster to the old. Take samples of the old plaster and determine its composition before beginning restoration.

Test Coats

• Applying a test coat can help plasterers determine whether the methods they've chosen will work in a given building. This coat should be applied over a small area of the surface, the same way the final plastering will be done. Observe the drying patterns of the plaster and look for weak spots, then remove the test coat. If problems occurred, it may be necessary to change the mix or technique slightly.

Keying

• Plaster that is applied directly over a previous coat will often fail to stick, unless that coat has first been keyed. Keying is the process of scratching lines into wet plaster, using a special tool called a lath scratcher, or simply the blade of a trowel. This produces a rough surface that more firmly attaches the next coat. Some plasterers believe that it's important to scratch out the joints between bricks and stones to provide a good key, but the Building Conservation Directory and Period Property Shop recommend against this practice. Plasterers who rake out pointing between bricks may find that their plaster dries unevenly and cracks easily.

Water

• When plaster needs to be applied directly to a clean brick wall, water can help it grip more effectively. Spray down the wall with clean water, allowing the brick to soak up as much as it can. The surface of the wall should be thoroughly damp, but without visible surface water. This damp surface allows the plaster to penetrate the brick more efficiently and to dry evenly. Plasterers should also wet wood lath before applying the first coat; this will take less water than brick.