Techniques for Sanding Wood

It has been said that the start makes the finish and sanding wood is the start of a beautiful finish. Before any wood stain or clear finish can be applied you must properly prepare the surface. Every professional painter and refinisher has his or her own preferences when sanding before the application of stain and clear finish.

There are a couple of good reasons for sanding wood that go beyond simply smoothing a surface.

• New wood trim can have a shiny look. This is called mill glaze. The mill process that created the wood trim profile causes mill glaze. This must be removed in order for the stain or finish to penetrate properly.
• Chemical stripping always leaves a little finish behind. Sanding is the only way to remove all remaining finish.
• Scraping and the use of a wood plane will close the pores of wood by crushing the fibers. Sanding the wood opens these pores, helping the finish to penetrate.
• Sanding creates tiny, invisible scratches on the surface that enhance the bond of non-penetrating finishes.

Sanding Wood for a Smooth Finish

Two types of sanding can be used; typically both are used on large jobs, hand sanding and the use of sanding machines.

Power sanders, such as palm sanders and orbital sanders, will cut through ruff wood and old finishes quickly but leave swirls in the wood. These grooves will show when stain is applied. Any vibrating sander will leave these grooves on the surface.

No matter which type of power sander is used, you will still need to do some hand sanding. Sanding wood with power sanders can't provide the smoothness and quality that only hand sanding can.

Folding the Sand Paper

A trick used by professional painters is to fold the sand paper into thirds. This will provide three sanding surfaces ready to be used. When one side becomes worn or overloaded simply refold with a new sanding surface showing.

When sanding by hand, use a sanding block to evenly spread out the pressure over a large area and speed up the work. The block also prevents you from sanding in one small area creating a depression in the woods surface. A ridged sanding block and paper will work for large flat areas, but contoured surfaces will need different tactics.

Most types of wood trim have an area of contoured profile that can't be sanded with a stiff block. Fortunately this is easily taken care of with a sanding sponge or a small piece of sand paper wrapped around your finger or a wooden dowel. The sponge works great and is available in a variety of grits.

No matter what tool or technique you use, always start sanding wood with the smoothest usable grit of sanding material and work your way through the different grits until the desired smoothness is reached.

If you start with a coarser grit than necessary, you will cut small grooves into the surface. These have to be removed in subsequent sanding. This means more work for you and less chance of success with your finishing project.

Coarse grit, such as 80 or 100 grit, can leave the surface rougher than smoother. In most cases start with 120 grit and finish with 150 grit. Keep progressing through finer grits until the desired smoothness is achieved. The final sanding grit depends on the type of finish you want to apply and the final look you want it to display. The final appearance of most wood finishes, varnish and lacquer, are unaffected by how smooth the surface is, as long as all visible scratches and defects are removed. The liquid flows over the surface filling small irregularities.

An ultra smooth surface can have an adverse affect on the clear wood finish. Most wood finishes form mechanical bonds with the wood. The smoother the surface the weaker these bonds become. For this reason the task of sanding wood should be stopped at 150-180 grit. This rule applies to finishes that form a thick protective coating, such as varnish or lacquer.

Penetrating finishes, such as Tung oil and Danish oil, are not affected by a smooth surface. Plus, a ultra smooth surface is desirable with this type of finish. Penetrating oils can't be easily sanded between coats, so it is best to achieve a smooth surface before application.

Another consideration is if you plan on using a wood stain, particularly the pigmented oil stains. The pigments collect in the tiny pores and scratches in the woods surface. The deeper the scratches, the more pigment accumulates in them and the scratches will appear more prominent. Usually sanding wood beyond 180 grit isn't necessary. You can experiment on a scrap piece of wood to see the end result and continue sanding with even finer grit if necessary.

Tips for Sanding Wood

• Refold or shift the sandpaper on the block often to expose fresh cutting surfaces.
• Vacuum or brush away the dust as you sand. This will keep the dust from loading the sandpaper.
• Before continuing with a finer grit vacuum the surface. This will remove the dust and any abrasives that have come loose from the previous sanding.
• Sand with the grain never against the woods grain. Use long even strokes, over lapping slightly.
• When the sandpaper is no longer cutting the surface effectively, discard it and get a new piece. Sanding wood can take considerable effort, new sandpaper is inexpensive compared to your effort.