Untreated lumber is wood that is not pressure treated for the outdoors. Pressure treated lumber undergoes water and other treatments to prepare it to be outside. Treated lumber is often used in decks and railings. It can also be used for flower beds. It is possible to use untreated lumber outdoors. When exposed to the rain and elements, however, it will rot and decay. Before using untreated lumber outdoors, it is necessary to prime and paint it. This will help seal it and prevent rot. Painting untreated lumber is just like painting any other type of wood.
Lay down a tarp and set each piece of untreated lumber that needs to be painted on top of the tarp. Sand down each piece with 100-grit sandpaper. Move in a smooth, even motion over both sides of the lumber. Turn it over and get the sides, top and bottom.
Sand until all the splinters and bumps on the lumber are smoothed out, and wipe off all of the dust from the sandpaper with a clean rag.
Open a can of exterior wood primer and dip a paint brush in it. Apply the wood primer to both sides of the lumber. Brush it on and move up and down each piece until the top and sides are covered. Wait one hour for the first side to dry. Turn it over and paint the other side of each piece.
Apply oil-based paint over the primer once the primer has had one hour to dry. Spread it out with a paint brush just like the primer was applied. Cover the entire area with an even layer of paint and wait one hour for the first coat to dry.
Inspect the lumber and look for any spots that are not covered, particularly where there are knots or other cracks. Add a second coat to further cover all those spots, if necessary. Let the lumber dry overnight before using it outdoors.