10.1: Preparing Wood for Finish

10.1 Preparing Wood for Finish

A finish can change the color, depth, and appearance of wood, but it does not hide poor surface preparation.

In fact, finish often makes scratches, glue residue, dents, and other defects more visible.

Good finishing begins before the finish container is opened.

Learning Outcomes

After completing this section, you should be able to:

• Inspect a project before finishing.
• Prepare wood through an appropriate sanding sequence.
• Explain why sanding scratches must be removed before moving to a finer grit.
• Raise the grain before final surface preparation when appropriate.
• Remove sanding dust before applying finish.
• Recognize common surface problems that become visible after finishing.

Finish Starts With the Surface

Before applying finish, inspect the entire project.

Look for:

• machine marks;
• sanding scratches;
• glue residue;
• dents;
• torn grain;
• rough end grain;
• pencil marks;
• sharp edges that should be softened;
• uneven curves; and
• areas that were difficult to reach during earlier sanding.

Correct these problems before finishing.

Once finish is applied, many of them become harder to repair.

Sand Through the Grits

Use a logical sanding progression.

A common WMT sequence may include:

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

You may not need to begin with the coarsest grit if the surface is already well prepared.

Each grit has one main job: remove the scratches left by the previous grit.

Do not move to a finer abrasive until the previous scratches and tool marks are gone.

Skipping too many grits usually creates more work rather than saving time.

Sand With the Grain

Final hand sanding on visible flat surfaces should generally follow the direction of the grain.

Cross-grain scratches can be difficult to see on unfinished wood but become obvious after oil is applied.

Use good lighting and inspect the surface from several angles.

Pay Attention to End Grain

End grain often absorbs finish differently from face or edge grain.

It can also become noticeably darker if it remains rougher than the surrounding surfaces.

Sand end grain carefully and consistently.

Do not assume that because an end feels smooth, it has been prepared to the same level as the rest of the project.

Remove Glue Completely

Dried glue can prevent finish from penetrating the wood evenly.

A glue spot that is nearly invisible before finishing may appear as a pale or uneven area after finish is applied.

Inspect carefully around:

• glue joints;
• inside corners;
• clamped areas; and
• places where squeeze-out occurred.

Remove glue without damaging the surrounding wood.

Raise the Grain

Water can cause small wood fibers at the surface to swell and stand up.

Before final finishing, your instructor may have you raise the grain.

A basic process is:

1. Lightly dampen the wood with clean water.
2. Wet the surface evenly rather than soaking it.
3. Allow the wood to dry completely.
4. Lightly sand the raised fibers with the final abrasive grit.
5. Remove the sanding dust.

The goal is to cut off the raised fibers without significantly reshaping the surface.

Let the Wood Dry

Do not apply finish to wood that is still damp from grain raising or cleaning.

Allow the project to dry thoroughly.

This is especially important if water, a damp cloth, or compressed air that may contain moisture has been used during final preparation.

When in doubt, give the project more time to dry.

Remove the Dust

Sanding dust left on the project can become mixed into the finish.

Before finishing:

• vacuum the project when appropriate;
• use a clean brush;
• use compressed air only as directed;
• wipe surfaces using the method demonstrated by your instructor; and
• inspect corners, grooves, joints, and other areas where dust collects.

Compressed air can move dust from one place to another, so use it thoughtfully and follow shop procedures.

Inspect Under Good Light

Before applying finish, look across the surface rather than only straight down at it.

Low-angle light can reveal:

• scratches;
• glue;
• uneven sanding;
• dents; and
• surface waves.

Your hands are useful too.

Run your fingers across curves and transitions.

A surface can sometimes feel wrong before it looks wrong.

Final Surface Check

Before finishing, ask:

• Are all machine marks gone?
• Are scratches from earlier grits gone?
• Is all glue removed?
• Are the edges treated consistently?
• Is the end grain properly prepared?
• Has the raised grain been removed?
• Is the wood completely dry?
• Is the project free of sanding dust?

If the answer is yes, you are ready for finish.

Check Yourself

1. Why does finish often make surface defects more noticeable?
2. What should each sanding grit remove?
3. Why should you avoid skipping too many grits?
4. Why can dried glue create a problem during finishing?
5. What does it mean to raise the grain?
6. Why must the wood dry completely before finish is applied?
7. What should you inspect before opening the finish container?