approach sanding from the perspective of the finish and not your finger

Understand what your finish system requires. You have to approach sanding from the perspective of the finish and not your finger. Of course you sand to level joints and smooth the surface; however you are also providing a uniform texture that will help take stains evenly and provide a tooth for the finish to grab on to. Selecting the correct grit and type of paper are crucial. Many of the high solids finishes require less white wood sanding than their older counterparts. Over-sanding is not only counterproductive, but it will ultimately encourage adhesion problems. MDF requires a different sanding schedule and mineral type than wood.

Wood should be sanded just prior to staining and finishing. Pieces sanded the day before could be subject to adhesion problems. If you can not sand and finish the same day, give a quick scuff to the surface the day you finish. Be sure to let coats of finish dry sufficiently before sanding, but don’t wait too long. The process of spraying a coat of finish, sanding it and then applying the next coat should optimally take place within a 24 hour period. The goal is to apply the next coat before the previous coat of finish is totally shrunk; this way the new coat is deposited in the sanding scratches of the first coat which will then shrink around the new material and give you maximum inter-coat adhesion. This is very important when using any type of catalyzed finish since they do not melt into the previous coat.

SANDING - HARDWOOD LUMBER AND VENEER

Sand unfinished wood with an open coat aluminum oxide paper, silicone carbide is too sharp.
Sand lumber with a grit no higher than a 150.
You can sand veneers one grit higher than lumber; but no higher than a 180 grit.
Use a progression of grits. Skipping a grit may cause uneven staining or even scratches to telegraph thru your topcoats.
Break edges. The finish film shrinks as it dries. Sharp edges will rip the film and allow water and oil to eventually get in. Think of it as stretching a piece of Plastic Wrap over a knife blade.
The day that you sand your wood is the day that you should stain and seal it. Simple things like changes in the humidity level can cause the wood to swell and you will lose the tooth you were trying to create.

SANDING - MDF

MDF faces should be sanded with a 320 silicone carbide paper. Silicone Carbide paper works the best since it is very sharp and the face of MDF is very hard, almost tempered.
MDF edges and profiles should be sanded with only 400 and 600 grit silicone carbide sandpaper. A course paper tends to pull out the wood fibers from the binder in the MDF, resulting in more pits in the surface. Use a course paper only when it is necessary to remove chatter from a cutter.

SANDING - BETWEEN COATS OF FINISH

Just as we sand to create scratches in the wood to give the surface a “tooth” for your stains and finishes to grab on to, we also need to create a texture, or scratch, between coats of finish so that the next layer has something to grip. Remember that only the old nitrocellulose lacquers melt into the previous coat of finish. All catalyzed finishes bond to the previous layer by a mechanical link; they need a rough surface. You should sand between coats of finish with 220 to 320 paper .A silicone carbide or a high quality stearated aluminum oxide paper, like the Mirka Royal, is recommended.

sanding guidelines

Your sander or hand block should have a soft pad. A hard pad may sand faster, but a soft pad will compensate for minor irregularities in the wood and will allow a more consistent contact of the grit with the surface it is sanding.
If you sand by hand, you should duplicate your style and movements consistently from piece to piece, edge to edge.
If you spot sand by hand use at least one grit finer than the last grit you used on the machine.
The particles on your sandpaper wear down or break off as they are used. A worn out 120 grit belt will polish a board smoother than a 400 grit belt. By replacing sandpaper or belts before they get too worn you will maintain a more consistent scratch pattern and depth.
Pieces should be recoated within 8 hours of being sanded.
To sand between coats of finish use a stearated silicone carbide paper or a high quality aluminum oxide paper, like Mirka Royal for sanding finishes.
Waterborne finishes will often develop fisheyes if sanded with a zinc stearated paper. Some manufacturers use a calcium stearated which will not cause fisheyes..
Woven pads, like Mirka’s Mirlon and 3M’s ScotchBrite are good for scuffing toner and washcoats. They scuff the surface, but won’t level it.
Avoid using Steel Wool as it may contain oil and the fibers are hard to clean up.
Avoid burn thru’s between coats. Give a burn thru extra drying time before recoating. This will reduce the chance of wrinkling the next coat of finish.
If you are going to glaze, remember that the glaze will stick in your sanding scratches so sand carefully.
Always remove sanding dust before recoating. Take extra care to get it out of the pores of woods like oak.

SWIRL MARKS

A word about swirl marks. If you have them, then there is a problem with your sander, your system or the person using it. An adequately powered, properly maintained sander, using the correct progression of grits will not swirl. Notice the keywords: adequately powered, properly maintained and correct progression of grits. A few simple precautions and you can all but eliminate them from your life.
If you are using pneumatic sanders, check your plumbing. Be aware that the length and size of piping and hose will affect performance. A common culprit is the use of reducers to go in and out of improperly sized filters or regulators.
Use Hi-Flow connectors to increase your CFM delivery to your machine by almost 100%.
Check your equipment. Inspect, Clean, Lubricate, Repair, Replace. Nothing lasts forever.
Check pads and bearings. Flat pads and smooth bearings; it’s a good thing.
Check your technique. Let the tool do the work. If you slow it down, it will swirl. Excess pressure on a sander will change a random orbital pattern into an orbital pattern.
Check your sanding schedule. Don’t make big jumps between grits. Skip steps and it will swirl.
Check your paper. Keep it sharp.
Micron graded paper will swirl less than CAMI or P graded papers.
Check the moisture content of your wood. If it’s wet, it will swirl.
Use your instincts. If it doesn’t look, feel or sound right, it will swirl.

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