New or restored surfaces:
Sand in order to remove raised wood fibers, any traces of resin should be removed using a nitro thinner. Fill any imperfections, sand and remove dust.
Give a first coat of filler diluted in 10 - 20 % water.
After drying, sand and apply a second coat for indoor applications, 3 coats for outdoor applications.
Already painted surfaces:
Remove old layers of paints that are peeling off and roughen the surface.
Sand any fillings and remove dust. Apply the primer as indicated above.