When a roof leak reaches the living space of your home, it stains the drywall where it soaks through the ceiling. Many people assume that they can just roll a coat of ceiling paint over the spot, but it just doesn’t work that easily because that stain will bleed back through ordinary paint.
The solution is a stain blocking primer, and the easiest way to apply it is with a spray can. In fact some manufacturers design cans to spray upward specifically for this purpose.
First, cover the area beneath the stain with a drop cloth, then gently dust the spot with the primer until it disappears. You want to feather the primer out a bit, so that the stain and the repair is less visible. And remember, several light coats are a lot better than one heavy coat.