How to Choose the Right Sandpaper Grit: A Practical Guide for Wood, Plastic, Resin, Metal, and 3D Prints

During an on-location photo shoot, we noticed a few fine sanding marks on the surface of one of our samples. They were barely visible under normal light, but once the lighting was set for the camera, every mark stood out.

The paper could not remove deeper scratches, but the sample was already mostly smooth. That final pass reduced the fine marks and dull, uneven appearance enough for us to finish the shoot.

The experience raised a practical question: when working with wood, plastic, resin, metal, or 3D prints, which sandpaper grit should you start with, and where should you stop?

The answer is not simply to choose the highest grit available. The result depends on the defect you need to remove, the material you are sanding, and what you plan to do with the surface afterward.

What Sandpaper Grit Actually Means

Lower grit numbers indicate larger abrasive particles. They remove material faster but leave deeper scratches. Higher grit numbers use smaller abrasive particles, making them better suited for removing the marks left by the previous sanding stage and creating a more refined surface.

Grit is not simply a count of how many abrasive particles fit into one square inch. More accurately, it refers to a classified range of abrasive particle sizes. 

There is one complication: sandpaper may be graded according to FEPA, CAMI/ANSI, or JIS standards. At coarser grades, the differences are usually limited. At finer grades, however, the same grit number may not represent exactly the same particle size.

For consistent results, use abrasives from the same product range throughout a project. If surface tolerances are critical, compare the specified particle size in microns instead of relying on the grit number alone.

What Each Grit Range Is Used For

These ranges are starting points, not fixed formulas.

Choose the finest grit that can still remove the defect efficiently. If 240 grit removes the scratch, there is no reason to begin at 120 grit. Starting too coarse may create deeper marks and add several unnecessary steps to the job.

Wood: Most Projects Do Not Need 1500 Grit

For wood that has already been cut, planed, or roughly shaped, 120 grit is a practical starting point for removing light tool marks and minor unevenness. Follow it with 180 grit to prepare the surface for most common finishes.

A simple sequence for bare wood is:

120 grit → 180 grit

The final passes should follow the direction of the grain. Cross-grain scratches may be difficult to see on unfinished wood, but they often become much more noticeable after stain, oil, or clear finish is applied.

Finer is not always better. Over-sanding can burnish the surface and affect how evenly a stain penetrates. 

Plastic: Heat Control Matters More Than Chasing a Higher Grit

“Plastic” covers a wide range of materials with very different hardness and heat resistance. For a hard plastic such as PMMA acrylic, 120 or 180 grit may be useful for correcting a rough-cut edge. For ordinary scratches, however, it is usually better to begin at 240 or 400 grit.

A practical sequence is:

240 grit → 400 grit → 600 grit → 800 grit

For a clearer or more polished surface, continue with 1000, 1200, and 1500 grit before applying a suitable plastic polishing compound.

PLEXIGLAS recommends sanding in progressive stages and using wet sanding where appropriate. Water helps reduce clogging and carries away heat generated by friction. Its official acrylic fabrication guide provides further guidance.

Excessive pressure is one of the most common mistakes when sanding plastic. Friction can soften the material, load the abrasive, or cause local deformation. Whether sanding by hand or with a powered tool, use light pressure and keep the abrasive moving.

Resin: Leveling and Polishing Require Different Sequences

For fully cured epoxy resin, begin with 120 or 180 grit when removing raised seams, drips, or obvious high spots. Use 240 and 400 grit for general leveling, followed by 600 or 800 grit to reduce the remaining scratch pattern.

For a high-gloss finish, continue with:

800 grit → 1000 grit → 1200 grit → 1500 grit

A 1500-grit abrasive creates a more uniform surface for polishing, but it may not produce a mirror finish on its own. The final gloss usually comes from a polishing compound, a resin-compatible finishing product, or a clear coating.

Before sanding, make sure the resin has cured completely. If it still feels soft or tacky, sanding will clog the abrasive and may damage the surface. Some epoxy systems also leave a waxy film after curing, so follow the resin manufacturer’s cleaning instructions before beginning.

Metal: Better for Refining Than Heavy Rust Removal

Within this grit range, 120 grit can remove light rust, small burrs, and noticeable surface scratches. Continue with 180 and 240 grit for leveling, then move to 400–800 grit for a more uniform satin or brushed appearance.

A useful starting sequence is:

120 grit → 180 grit → 240 grit → 400 grit

For a smoother finish, continue with 600 and 800 grit. If the goal is a polished surface, follow with 1000, 1200, and 1500 grit before using a metal polishing compound.

When creating a brushed finish, sanding direction is just as important as grit. Keep each pass moving in the same direction. Crossed sanding lines are easy to see when light reflects from the metal.

Soft metals such as aluminum can clog an abrasive quickly. Clean the workpiece and abrasive regularly instead of continuing to sand with trapped metal particles.

3D Prints: Match the Starting Grit to the Printing Process

FDM prints often have visible layer lines and support marks. Begin with 120 grit on raised areas, then work through 180, 240, and 400 grit to level the surface.

A practical FDM sequence is:

120 grit → 180 grit → 240 grit → 400 grit → 600 grit

If the part will be painted, apply a thin coat of primer after the first leveling stages. Primer makes remaining layer lines, low spots, and small defects easier to see. These areas can then be corrected with 400- or 600-grit abrasive before the final coat.

Thermoplastics such as PLA and ABS are sensitive to friction heat. Powered sanding may save time, but it can also round over sharp edges, erase small details, or soften the surface. Use light pressure and avoid staying in one place.

SLA resin prints generally have finer layer lines. Support marks can often be treated starting at 240 or 400 grit, followed by 600 and 800 grit. Display models and parts intended for a smoother finish can then be refined with 1000, 1200, and 1500 grit.

Do Not Move to the Next Grit Too Soon

Sanding works by replacing deeper scratches with progressively finer ones. Before changing abrasives, the current grit should remove the scratch pattern left by the previous stage.

Do not rely on touch alone. A surface can feel smooth while still containing deep sanding marks. Clean the part and inspect it under low-angle light. If isolated deep scratches, glossy low spots, or uneven areas remain, continue with the current grit.

Here are practical starting sequences for common materials:

• Wood: 120 grit → 180 grit

• Plastic: 240 grit → 400 grit → 600 grit → 800 grit

• Resin: 240 grit → 400 grit → 600 grit → 800 grit → 1000 grit → 1200 grit → 1500 grit

• Metal: 120 grit → 180 grit → 240 grit → 400 grit

• FDM prints: 120 grit → 180 grit → 240 grit → 400 grit → 600 grit

• SLA prints: 240 grit → 400 grit → 600 grit → 800 grit → 1000 grit → 1200 grit → 1500 grit

You do not have to use every grade on every project. Start in the middle if the original surface is already smooth, and stop early if the next process does not require a high-gloss finish.

Figure 5. Inspecting the finished model under low-angle side lighting to reveal any remaining scratches.

The Rule That Makes Grit Selection Easier

Before choosing a sandpaper grit, answer two questions:

What needs to be removed, and what will happen to the surface next?

Begin with the finest grit that can remove the current defect efficiently. Move to the next grade only after the previous scratch pattern has disappeared. This approach reduces rework, protects fine details, and prevents unnecessary damage caused by starting too coarse or sanding far beyond what the project actually needs.