Mix a small amount of Venetian plaster in a disposable container. This medium sets very quickly. Only use enough to cover a small area. You can make more as necessary.
Apply the plaster to the wall with a trowel in a cross hatch pattern. Slide the trowel up the wall in one direction. Go back to the bottom where you started and slide it up going the opposite way. Do not worry about ridges -- this is part of the charm of rustic plaster walls. Create your own unique pattern as you work, leaving as many ridges, or spots, as you would like. Let the plaster set until it is hard -- usually about 45 minutes.
Paint over the plaster with water-based latex paint. Use the color of your choice and apply the paint with a paintbrush to get into the nooks and crannies created by your trowel. A roller will not work. Let the paint dry for four to six hours.
Apply a glaze over the painted plaster with a paintbrush. Apply more in some areas, and less in others. Let this sit for five to 10 minutes.
Wipe off the glaze with a damp sponge. The glaze will adhere to the ridges in the plaster, highlighting these spots.