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Even the greatest finish job will look bad without the proper surface preparation.
Unfinished wood preparation is usually not regarded as the most exciting step in finishing, but knowing this process will greatly affect the final outcome of the finish makes it a worthwhile one. Making sure the wood parts are properly prepared is the first step to an outstanding finish — even if such preparation takes place in another department. The preparation between finish steps is equally as important.
A sticky situation
Preventing or minimizing glue on the surface to be finished is very important. Glue marks that show up after staining can be difficult to touch up in the finishing department. If glue does get on a substrate to be finished, it must be removed completely to prevent a discolored area during finishing. Depending on the type of glue, removing it while it is still wet and sanding the surface after it dries may be the best method. If such glue issues are not taken care of by the time it reaches the finishing department, this may mean the part has to be taken off-line and corrected before being placed back on the line. For example, if the glue dries, it may need to be chipped off and the area sanded, consuming time, labor and money.
Watch your sandpaper
Sandpaper grit can greatly influence a perfect finish. The coarser the grit, the darker a wiping stain will take. The finer the grit, the lighter a wiping stain will take. Coarse grit sandpaper also is more likely to show sanding marks and must be evaluated for each wood and finish type to make sure the sanding marks are not apparent. Pigmented gloss finishes show sanding marks easily, and great care must be used to get a smooth surface.
Too fine a grit of sandpaper can interfere with adhesion of the finishes. This is especially apparent on woods such as hard maple that have been sanded with too fine a grit of sandpaper. The finish may not be able to get a hold of the wood and could chip or crack easily. Wood finishes adhere to wood by “mechanical adhesion”; the finish wraps around the top wood fibers and holds on to them. If there are not enough fibers to grab, the adhesion can be poor. Wood substrates are normally sanded at a 120 to 180 grit, depending on the wood specie and the type of finish.
Sanding equipment
Sanding equipment often leaves a swirl scratch pattern on the wood surface. Using an in-line sander can help prevent this. Objectionable scratch patterns can be caused by starting or stopping the sanding improperly, torn sand-paper, a foreign grit trapped under the sander, etc. Check thoroughly for the cause of the scratches so the problem can be corrected quickly. It may be necessary to block sand by hand, in line with the grain to remove problem scratches. Only sand enough to remove the objectionable marks. This is especially important on veneers since it is easy to sand through, ruining the part. Your equipment and abrasive suppliers can help optimize your process.
Other considerations
Wood can have mineral streaks or discolorations. Use wood bleach to lighten discolored areas, making it easier to achieve the correct color. Lighter areas can be darkened by applying a “sap” spray stain before finishing.
It is necessary to closely inspect the wood substrate under bright lights before finishing to find any defects such as glue marks, water marks, dents, gouges, veneer seams, sanded through veneers, poor fit of parts, mineral marks and discolorations in the wood. Correcting these problems before hand will prevent touch-up or refinishing later.
Between steps
The preparation necessary between finish applications is equally important as preparing the substrate before finishing.
