Have you ever looked at a gilded wall or ceiling and been able to see the "start/stop" lines? This is usually caused by gilding on all available size, then resizing by cutting in to the gild mark. It will always leave a mark or a line, (usually a thicker application at the cut mark).
If doing a gilded ceiling with composite leaf, using a waterbase size such as Rolco Aqua size works fine. In my experience the best way to apply this size is to spray it. Do the whole ceiling at once. This way any start/stop lines will not happen and spraying allows for the smoothest application.
If doing walls or ceilings using real gold, oil size is my choice. First, on a wall it is just more durable as everyone is going to want to touch it. Usually when your not looking! Not only for durability, but with real gold it just allows it a better brightness. If its a large area and sizing the whole surface isn't practical for fear of loosing tack, size out about a half foot more area than you plan to gild. Once your gilding is up to that point, size for the next day by rolling just into that half foot area blending it smooth. And make sure you size an extra half foot, 4-5 inches is fine too but at least the width of a leaf. Just make sure you don't gild all your size and you should have no problem.
