Enough primer should be applied to obscure the wood grain. Two topcoats are applied over the primer. If it is not practical to double-coat the entire house, then consider two coats on the south and west sides, because they weather more quickly. All-acrylic latex paints are easy to use and very easy to clean up with water afterwards, with no potential risk of fire hazard from oily rags, thinners, etc. Most professionals and do-it yourselfers prefer it.
Oil adheres better, but preparation and cleanup discourages many.
Oil alkyd is faster drying, but presents the same problems as oil. Use a dry brush to remove dust, particles of wood and other foreign matter before you paint. Seal all cracks around doors and windows, and wherever wood comes together, with caulking compound to prevent moisture problems. To minimize moisture problems, we recommend that fans be installed in kitchens, bath and laundry rooms. Attics should be ventilated, crawl spaces covered with 3 or 4 mil polyethylene film and the area properly vented.