Whatever coating you plan to use as a final baseboard finish, proper sanding guarantees a smooth surface that's ready for brush or spray applications. The sanding step is an opportunity for you to fill unsightly nicks, cracks and nail holes adding to the quality touches of the room. Sanding also roughs an existing finish or a new, unfinished surface to better accept the new coat of paint or varnish.
Remove paint flakes and sand marred areas before you use wood filler for better adhesion. Press filler into nicks and cracks with a putty knife, overfilling them slightly, as the filler may shrink as it dries. Pay particular attention to inside and outside corners where pieces of baseboard meet. Wood filler helps mask minor signs of house settling as well as hides imperfect miters. Allow the filler to dry for several hours or overnight before sanding.
Sand lengthwise along the baseboard in the direction of the wood grain using long strokes. Wrap a flat sanding block with medium-grit sandpaper for flat areas of the baseboard: 60 to 100 grit is good for initial sanding. Finish the surface with 120- to 180-grit fine sandpaper to achieve satiny smoothness. A flexible sanding block made with a rubber or foam core conforms exactly to the contours of baseboard molding. This type of block is available already covered with various grit coatings. Buy blocks with the appropriate grit to handle the condition of your baseboard moldings.
Vacuum the dust frequently as you work. Use a brush attachment to vacuum the sanded baseboard surface, then wipe the baseboard with a tack cloth to remove all traces of dust. The surface is ready for paint, stain or varnish.
Set up sawhorses or a workbench to accommodate the baseboard pieces. Do this outdoors, if possible, to help with dust control.
Place sections of baseboard on the work surface, securing each with clamps to hold it steady as you work on it. With a sanding block and fine grit sandpaper, sand lengthwise with the wood grain. New, unfinished baseboard is usually ready for finish sanding.
Vacuum the dust from the baseboard and follow with a tack cloth. Paint or varnish the baseboard before installation, keeping sanding supplies handy for touch-ups as you cut the trim.