Scrape and clean the wood with a paint scraper, rag and mineral spirits. Sometimes knots in wood can ooze sap and this needs to be cleaned before any primers or paints are applied.
Fill any cracks or holes in the knots with stainable wood filler and a putty knife. Stainable wood filler can be found at hardware stores.
Sand the wood with coarse-grit sandpaper, followed by fine-grit sandpaper. Remove all dust and debris with a tack cloth.
Apply two coats of a clear amber shellac or polyurethane using a wide paintbrush. Allow each coat to dry according to manufacturer's specifications.
Apply one coat of white pigmented shellac using a wide paintbrush. This will prepare the piece of wood for the primer application. Allow the paint to dry according to the manufacturer's specifications printed on the packaging.
Apply one coat of oil-based primer using a sponge roller and paint tray. Allow the primer to dry thoroughly. Follow with two coats of oil-based paint in the color of your choice. Allow the paint to dry.