This story covers painting house siding.
After scraping away loose paint and repairing damaged sections, spot-prime any bare wood with a tannin-blocking primer. Work the primer into the wood, taking care to not apply it too thin. Primer is not paint, however, so you don't need to apply it so thickly that it completely hides the surface underneath it.
Let the spot-primed areas dry (they will dry quickly). If the remaining surface is painted and the paint is well adhered, you can proceed directly to painting it. If the coating is in poor condition or you're working with unpainted wood, you'll have to prime it. Brush prime all the edges of each board. Don't edge-prime so large an area that the primer dries. Use a roller to fill in the face of the boards while the edges are still wet.
Before the paint on the edges has dried, fill in the face of the boards with a brush or 7- or 9-inch medium-nap roller, smoothing out any bead marks but using care not to over-roll the paint or exceed the spread rate recommended by the manufacturer. If you're applying two coats, let the first coat dry as specified on the paint can label and repeat the process with the second coat.