Understanding how to sand wood begins with selecting the proper sandpaper grits and following a few basic steps. Whether wood is unfinished or has been previously painted, the sanding motion should be in the same direction as the wood grain.
Sanding painted wood offers two important benefits. The first advantage is that it can transform an existing rough, irregular painted surface into a smooth, professional looking surface. Sanding hard painted finishes also improves adhesion for new coats of paint.
Sanding unfinished wood is the only way to eliminate unsightly saw marks and planer marks. Sharp corners (edges) can also be slightly eased so the finish will cover. Because sanding creates dust, some care should be taken to mask and cover anything that is intended to stay clean.
Choosing a Sandpaper Grit
Knowing how to sand wood requires choosing the right sandpaper grits.
Sandpapers are graded according to coarseness. The grit number is calculated by the number of abrasive particles per square inch. The lower the grit number, the coarser is the sandpaper. Conversely the higher the grit number, the finer is the sandpaper.
In order to properly choose the correct coarseness of sandpaper you should first evaluate the condition of the existing surface.
80-100 grit: If the existing paint is generally in poor condition, 80-100 grit, coarse sandpaper is a good starting point. Peeling and flaking paint should first be scraped. Once the loose paint has been removed, the paint edges should be feather sanded to remove uneven ridges. Thick, uneven paint, such as that caused by paint runs, dirty paint, and very heavy brush marks or roller texture, can be sanded smooth.
Coarse sandpaper can be used to sand out saw lines and planer marks on unfinished wood intended to be stained or painted.
Sanding with coarse sandpaper such as 80grit or 100grit will leave scratches on unfinished wood and painted wood surfaces. Sanding with increasingly finer sandpaper grits is a good way to achieve a smooth surface. Each, progressively finer grit will remove the scratches from the previous coarser grit. Follow up with 120 grit sandpaper for the 2nd sanding and 150 grit sandpaper for the final sanding.
120-180 grit: If an existing painted finish is in fair to good condition, use medium grit sandpaper, such as 120grit to 150grit. Either of these grits works well for sanding spackle and hard filler, such as polyester glazing putty. Moderate brush marks or roller nap can also be sanded smooth
Medium grit sandpaper is also a good choice for preparing unfinished woodwork for paint or stain. If you start with 120grit sandpaper, use 150-180 grit sandpaper for the 2nd and final sanding. For soft woods such as Pine, Fir, and Redwood always use 180grit sandpaper for the final sanding.
220-320 grit: Fine sandpaper performs well sanding between new coats of paint and clear finishes on woodwork.
How to Sand Wood
Once the proper sandpaper grits have been selected, you must decide on the method of sanding. Random orbital sanders, palm sanders, and block sanders are well suited for flat surfaces. Sandpaper sheets and sanding sponges work well for irregular surfaces.
Hand sanding with sandpaper, sanding sponges, or sanding blocks is accomplished with a back and forth scouring action. A moderate amount of pressure must be applied while the sandpaper is in motion. The sanding direction should always be in the same direction as the wood grain, with very few exceptions.
Power sanding with a random orbital sander or a palm sander combines the elements of hand sanding with the high speed motion of the sanding pads. The rapid movement helps to scour surfaces more quickly. Even though orbital sanders are less likely to leave sanding scratches than palm sanders, utilizing the proper sandpaper grits is important.
Protect Adjacent Surfaces from Sanding Scratches
Learning how to sand wood also includes protecting all adjacent surfaces not intended to be sanded. Unintentional sanding scratches can result in costly damage. This is especially the case for windows, doors, and cabinets. Always use care when working around delicate surfaces.