Grit: Is identified by numbers from 600 to 12, the smaller the number the coarser the grit.
Density: "Closed-coat" indicates that the grit covers 100% of the surface, "open-coat" indicates 50% to 70% of the surface is covered. "Closed-coat" sandpapers are designed for fine finishing, "open coat" does not clog as easily so is best for initial sanding and paint removal.
Adhesives: Hide glue is used on paper for light or medium duty, it is not waterproof so cannot be used on a wet surface, waterproof resins are used on paper meant for wet sanding.
Basic Sanding Procedures
Always start with the finest grade that is usable, if the sandpaper is coarser than required small grooves will be made in the surface and will have to be removed with subsequent sandings creating more work for yourself.
150 grit will smooth the surface enough for painting, if a painted surface is too smooth the paint will not adhere as well.
Continue sanding for clear finishes moving down to finer and finer grits until the surface feels as smooth as you think you can get it. Now you will have to raise the grain, soak a clean cloth in water wring it out and dampen the sanded surface. Let dry for 24 hours, it will feel fuzzy to the touch, sand with fine sandpaper until smooth, this should be repeated until the wood no longer feels fuzzy when dry.
Open grained wood such as oak may need a wood filler to create a super smooth surface, followed by the application of a sanding sealer to prevent the final finish from lifting the filler. Sanding sealer also prevents the excessive absorption of stain on fine grained woods such as pine.
If sanding by hand a sanding block makes the job much easier, these can be purchased or made from pieces of wood, curved shapes can be sanded using pieces of dowel with the paper wrapped around it.
There are three types of power sanders, belt, disk and finishing sanders, the belt and disk styles are not suitable for finishing work. The finishing styles have straight line or orbital actions, some are a combination of both with a selection lever. Straight line sanding is slower than orbital but does a smoother job. When using a power sander let the weight of the tool do the work, do not push down on it.
A tack cloth is one of the best ways to remove dust and grit before applying a finish, to make one soak a cheese cloth in water, wring out the water, soak it in turpentine, wring it out again, drip enough clear varnish on the cheesecloth to make it evenly gummy throughout. Store it in a jar with a lid to keep it from drying out.
Mix one part wood glue with ten parts water, apply with brush, let dry. This will raise the grain, sand smooth.Save Your Sawdust
Before you start to sand a project empty the dust bag, when the bag is full empty it into a clean mustard or ketchup squeeze bottle. You will now have a supply of sawdust to match the project if you need to do the sawdust and glue fill. To do this squeeze some glue in the crack, then spray a bit of sawdust on it and work it in, repeat if necessary, remove excess.