On a fašade renovation job of an existing building, the success of application of plaster to an existing surface can vary greatly. In most cases a trial patch of an area of at least 3x3 feet is adviseable. If you make it to a smaller area, you may not see the effects of application to a large surface area. Usually you need to use mesh and attach it mechanically to the existing surface, otherwise there is a great risk of your new plaster layer not adhering sufficiently to the existing surface. It is best practice to involve a person who is expert at the fašade renovation as early as possible. Although you will pay him, this will most probably save you time and schedule down the road.