5.5: Abrasives and Sanding

5.5 Abrasives and Sanding

Sandpaper is not simply paper that scratches wood.

An abrasive is a cutting tool made from many small, hard particles attached to a backing.

Each particle removes a tiny amount of material.

Sanding is most effective when you treat it as a controlled cutting process rather than as endless rubbing.

Learning Outcomes

After completing this section, you should be able to:

• Explain what abrasive grit numbers mean.
• Choose an appropriate starting grit.
• Use a logical grit progression.
• Explain why skipping grits can leave scratches behind.
• Sand with and across grain appropriately during different stages.
• Identify the primary uses of common stationary sanders.
• Describe basic WMT stationary-sander safety requirements.

Understanding Grit

Abrasives are identified by grit number.

In general:

• lower numbers are coarser;
• higher numbers are finer.

Coarse abrasives remove material quickly and leave relatively deep scratches.

Fine abrasives remove less material and leave smaller scratches.

Start With the Right Grit

Do not automatically begin with the coarsest abrasive available.

Choose the coarsest grit that is actually necessary to remove the marks or irregularities on the surface.

If the surface is already well prepared, starting too coarse creates scratches that you then have to remove.

A Typical WMT Progression

A common progression might be:

P80 or P100 → P120 → P150 → P180 → P220 or P240 → P320

The exact starting grit depends on the condition of the work.

Do Not Skip Grits

Each grit should remove the scratches created by the previous grit.

If you skip too many steps, the finer abrasive may not efficiently remove the deeper scratches.

For example, moving directly from a very coarse grit to P320 may make the surface feel smoother while deep scratches remain underneath.

Those scratches often become much more visible after finish is applied.

The goal is not to use every grit ever made.

The goal is to progress in reasonable steps.

How Do You Know When to Change Grits?

Do not move to the next grit just because you have sanded for a certain amount of time.

Move on when:

• the previous tool marks are gone;
• the scratches are consistent with the current grit; and
• the shape is correct.

Inspect the work under good light.

A low-angle light can make scratches easier to see.

Sanding With the Grain

For final hand sanding on visible faces, sand with the grain.

Cross-grain scratches are especially noticeable after finish is applied.

Earlier shaping or machine sanding may not always follow the grain exactly, but the final hand-sanding passes should remove visible cross-grain scratches.

Use a Sanding Block

On flat surfaces, wrap the abrasive around a sanding block when appropriate.

A block:

• keeps the surface flatter;
• spreads pressure over a larger area; and
• reduces the tendency of your fingers to create grooves or rounded areas.

For curves, use a block or backing shape that matches the surface.

Keep the Shape

Sanding can change the form of a project.

Be careful not to:

• flatten a flowing curve;
• round over a crisp feature accidentally;
• make one edge thinner than another; or
• enlarge an opening unevenly.

The shape should already be correct before fine sanding begins.

Stationary Sanders

Stationary sanders remove material much faster than hand sanding.

They are useful for shaping and refining, but they can also ruin a part quickly if used carelessly.

Disc and Combination Sanders

A disc sander uses an abrasive disc mounted to a rotating plate.

Some machines also include a vertical sanding belt.

These machines are useful for:

• straight edges; and
• outside, or convex, curves.

They are not used to square stock or create precision joinery.

Disc-Sander Direction

Only use the downward-traveling side of the disc.

On that side, the disc pushes the workpiece down toward the table.

Using the upward-moving side can lift the work from the table and reduce control.

Disc/Belt Sander Safety

• Hold the work securely against the table.
• Maintain at least 2 inches between your fingers and the abrasive.
• Use moderate pressure.
• Keep the work moving.
• Do not force the work against the abrasive.
• Notify your instructor if a disc or belt is loose, damaged, or not tracking correctly.

Too much pressure creates heat, damages abrasives, and can make the work harder to control.

Combination stationary sander showing the abrasive disc, vertical sanding belt, work table, and power switch.

Figure: Disc and Combination Sander

Oscillating Spindle Sander

The spindle sander is designed primarily for inside, or concave, curves.

A cylindrical sanding sleeve rotates while moving up and down.

The oscillating motion spreads wear and heat over more of the abrasive surface.

Choose the Largest Practical Spindle

Use the largest spindle that comfortably fits the curve.

A spindle that is much smaller than the curve tends to create:

• uneven surfaces;
• small dips;
• more abrasive wear; and
• more work.

Spindle-Sander Practices

• Feed the work against the direction of spindle rotation.
• Use moderate pressure.
• Use the throat plate that leaves the smallest opening around the spindle.
• Hand-tighten the spindle as directed.
• Return spindles, throat plates, and tools to their proper locations.
• Notify your instructor when an abrasive sleeve is badly worn.

Figure: Oscillating Spindle Sander

Wide-Belt Sander

The wide-belt sander is used for finish smoothing flat material.

It is not intended to remove large amounts of wood like a planer.

When used in WMT, it should remove only small amounts of material through several light passes.

Students must be checked out on the machine before using it.

Use the planer first when substantial thickness must be removed.

Edge Sander

The edge sander is a long horizontal sanding belt used primarily for sanding edge grain.

It is useful for long edges and some outside shaping operations.

The edge sander removes material quickly, so use controlled pressure and keep the work moving.

Follow the instructor demonstration and WMT machine-specific limits whenever using it.

Power Sanders

Hand-held power sanders can speed up surface preparation, but they do not replace careful inspection.

Keep the sander moving and allow the abrasive to cut.

Excessive pressure:

• creates heat;
• reduces abrasive life;
• may produce uneven surfaces; and
• can leave visible sanding patterns.

Finish with appropriate hand sanding when necessary.

Before Moving On

Before declaring a surface ready for finish:

1. Inspect it under good light.
2. Look for scratches from the previous grit.
3. Feel the surface with your hand.
4. Check corners and transitions.
5. Make sure the shape has not changed unintentionally.
6. Remove sanding dust.

A finish does not hide poor sanding.

It usually makes poor sanding easier to see.

Check Yourself

1. What does a lower grit number indicate?
2. Why should you not skip too many grits?
3. When should you move to the next grit?
4. Why is a sanding block useful on flat surfaces?
5. Which side of a disc sander should be used?
6. Which stationary sander is best suited to inside curves?
7. Why should the largest practical spindle be used?
8. What is the purpose of a wide-belt sander?
9. Why can excessive pressure make sanding less effective?