Surface preparation & its impact on finishing: effective sanding is essential for a quality finish.
There are three commonly used methods to prepare the surface of the wood as a final step before finishing: widebelt sanding, random orbit sanding and brush sanding.
Widebelt sanding to 220-240 grit sandpaper works well for flat panel work and where a linear scratch pattern with the grain of the wood is acceptable. Random orbit sanding is used as a final step to remove minor defects and provides a scratch pattern that is invisible after finish is applied. Brush sanders use strips of abrasive and are backed by different types of brush media to force the abrasive strips into corners and recessed profiles of the substrate.
Of these three, brush sanding has become one of the most viable technologies to become lean and improve profitability in finishing. Brush sanding can provide the following process benefits: consistent breaking of sharp edges to eliminate sealer sand-through; consistent stain color and elimination of most blotchiness; elimination of most fiber raise, an important consideration with water-based finishing; removal of drag marks and minor defects; opens the pores of the wood uniformly for even finish absorption; and may reduce volume of coatings consumed.
Lean & Other Brush Benefits
From a lean perspective, brush sanders will reduce rework, lower sanding preparation labor and lower material costs. In addition, correct brush sanding technology will provide a return on investment quicker than most any other investment in finishing. The ROI for most manufacturers with $7 million to $10 million or more in sales should be less than one year, but not more than three years from purchase.
The benefits of finish quality cannot be overstated when using the right brush sanding procedure. The uniformity of stain colors, depth of finish and the clarity of the finish will dramatically improve the visual perception of the product. In some situations, a color step may be eliminated to reduce the amount of the cost in the finish, while still providing a high-end look.
Brush sanding will remove the following: knife marks above 14 per inch, dried marks and surface defects, excessive fiber raise, and blending of cross scratch and swirl marks. However, it cannot remove large indentations and defects, glue, chatter marks and heavy cross scratch on uneven joints.
The best results are achieved when abrasives have a long contact or dwell time on the part. Slower RPMs allow the abrasive to sand better down into the complex profiles. They also allow the sandpaper to remove the wood fiber, whereas a high speed may pull up fibers without sheering and removing them from the substrate.
Long abrasive strips will do a much more effective sanding job than short abrasives. Longer abrasive media will allow the strips to bend and sand along the surface of the strip rather than on the tip of the abrasive strip. Narrow strips work best for non-aggressive sealer sanding. Wide strips provide a more aggressive sanding action to remove deep defects.
It is important that all abrasives be broken in before placing them into production. This will ensure an invisible scratch pattern and provide a scratch profile that will allow the stain to develop to the correct color. It is also critical that an abrasive change-out procedure is followed for a consistent finish. Proper sanding preparation is the secret for successful finishing.
Phil Stevenson is president and owner of AWFI For information visit AWFI.org.
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|Title Annotation:||Sanding & Finishing|
|Comment:||Surface preparation & its impact on finishing: effective sanding is essential for a quality finish.(Sanding & Finishing)|
|Publication:||Wood & Wood Products|
|Date:||May 1, 2013|
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