Sanding wood: grit and technique

In short: Sanding makes wooden surfaces smooth and prepares them for oil, wood stain or varnish. You work your way from coarse to fine grit until the desired surface is reached.

In more detail: The grit of sandpaper is usually given as a P followed by a number: the higher the number, the finer the grit. If you skip grits or sand across the grain, you often only see the scratches after finishing.

In detail

• Coarse grit: Roughly P40 to P80 is used to remove old layers of paint or to level uneven surfaces.
• Medium grit: Roughly P100 to P150 removes the marks left by coarse sanding and prepares the surface.
• Fine grit: From about P180 to P240 you get a smooth surface ready for finishing; even finer grits are used for sanding between coats of varnish.
• Work in stages: When changing paper, do not skip far ahead in grit, because fine paper can hardly remove deep scratches.
• With the grain: When sanding by hand, always follow the direction of the grain; scratches running across it are especially noticeable after oiling or staining.
• Tools: A sanding block is enough for small areas; orbital or random orbital sanders suit large areas, and detail (delta) sanders suit corners.
• Raising the grain: Before water-based finishes you can lightly dampen the wood, let it dry and lightly sand off the raised fibres.
• Dust protection: Wood dust is harmful to health, and oak and beech dust is even classified as carcinogenic; so use dust extraction and wear a suitable dust mask, such as FFP2, and safety glasses.
• Old coatings: Very old layers of paint can contain harmful substances such as lead; if in doubt, do not dry-sand them and seek expert advice (holz-arten-und-eigenschaften, holz-leimen-und-verbinden).