Slate is often used to paint on because of its rough texture, which provides depth to anything painted on it. Because slate is naturally dark it also tends to make paintings stand out. Sheets of slate must be washed before they can be painted on. Any foreign materials left on the slate can interfere with the paint, and even cause sections of your artwork to flake off. Removing these contaminants is an easy but important step.
Rinse the surface of the slate with warm water to prepare it for washing.
Gently scrub the slate with soapy water (dish soap works well) and a scrub brush. If there are areas of caked on grime a bit more elbow grease will be required.
Wash large pieces of slate (or especially tough caked on debris) with a pressure washer, followed by a gentle scrub with soapy water.
Rinse all of the soap off of the slate surface.
Allow the slate to dry before painting.