How to Sand Compound Joints
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anding compound sawed joints to fit, and fitted joints in wood to finish it is somewhat different than flat sanding, care should be taken to observe the direction of the grains where the ends of separate boards meet. Here are some basic instructions for how to sand compound joints in wood.
Steps

Use a disk sander with an adjustable angle to sand the ends of your work pieces boards until the fit is acceptable. The purpose of this step is to assure a very close, tight fit in the ends of two boards cut on a compound angle, such as crown moulding or a shadow box picture frame. If you do not have access to a disk sander, you may fashion a sanding block by stapling a piece of medium grit sandpaper to a block of wood. The sanding process needs to be done in a flat manner to get a good fit.

  • Begin with the two boards an eighth of an inch or so longer than you need them, cut with a miter saw, or a little longer still if cut with a circular saw, since the cut will not be as precise.
  • Hold the two boards together at the correct angle for the finished fit, and scribe or mark any portion of the joint that is long, or interferes with an overall fit.
  • Mark the long portion of the cut end with a sharp pencil, doing so very lightly if the finished product will be natural finish.
  • Using the disk sander, position the table to support the work at the correct angle parallel to the mark. Sand in uniform passes, checking the two pieces for fit frequently.
  • When the fit is close enough, or satisfactory, fasten the pieces together. Some assemblies may use corrugated fasteners, or you may chose finish nails, wood screws, or glue. If you are equipped, you may wish to biscuit or dowel the joint, so it is very strong with no visible fasteners.

Get a random orbital disk sander to sand your finished product, or use a sanding block sanding. This will assure the sandpaper rests flat against the surface you are sanding.  Begin with a suitable sandpaper grit. If you are working finished wood which has been planed with a planer to a flat, smooth, finished surface you will only have to sand splintering at the cuts, and sanding to fit the joint. On the other hand, for rough sawn wood, you will want to begin with a coarse grit paper, about 100 grit, and work your way up in grit 150grit, then possibly 200 or 220 grit until the surface is smooth enough to stain, seal, or paint. Sand in the direction the grain is running. This is where a compound joint gets tricky, but sanding across, or perpendicular, to the grain will cause the grain to raise, or fluff up, making your job finishing the piece more difficult. With the random orbital sander, the sanding will be much smoother, and grain direction is not nearly so important.

Apply stain, paint, or sanding sealer, and allow to dry. If your work is rough, the finish you are using has raised the grain of the wood, and you will need to resand it before applying a final coat. Use a fine paper for this step, as you are not wanting to remove any excess wood in this step.
Tips
The closer the fit on the original cut, the less hand fitting will be required to make the final assembly satisfactory. Using a compound miter saw, properly aligned on the correct angles should produce a good fit with not sanding.
Warnings

Use of power tools can be dangerous.

Wear safety glasses and a dust mask while sawing and cutting.
Things You will Need

Lumber and saw.

Disk sander.

Random orbital sander.

Sandpaper in various grits.
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