A plane instead of sandpaper on a board cup

If you mean a cupped board, a hand plane can be a better choice than sandpaper when you need to flatten the face: it removes the high areas quickly. Sandpaper is gentler and useful for small corrections, but sanding a wide cup flat can take a long time. Either way, check how much thickness you can afford to lose before you start.

When a plane is the better choice {#when-a-plane-is-the-better-choice}

Choose a plane when the cup is noticeable and the board has enough thickness to flatten. A hand plane cuts wood rather than abrading it, so it can remove high spots more efficiently than sanding. It is also easier to control than a powered belt sander, which can dig grooves or leave a face uneven if it is tilted or held in one place.

A plane does not automatically make a board flat. A short plane can ride over low areas, and a dull iron or unfavorable grain direction can cause tear-out. For a broad face, a longer plane helps you check and work toward a consistent surface. If you do not have experience setting and sharpening a plane, practice on scrap first or consider having the board flattened by a woodworker.

When sandpaper makes more sense {#when-sandpaper-makes-more-sense}

Use sandpaper for slight cupping, small high spots, or final surface preparation when preserving thickness matters more than speed. For flattening, attach the abrasive to a long, rigid, flat sanding board rather than sanding with a small folded sheet or soft pad. A short or flexible backing can follow the cup instead of correcting it.

Start with an abrasive coarse enough to remove the high areas without excessive effort, then move to finer grits to remove the earlier scratches. The right starting grit depends on the wood, the amount of material to remove, and the finish you plan to apply. Check progress often with a straightedge across the face; do not assume that a smoother-looking surface is flat.

A careful way to flatten the board {#a-careful-way-to-flatten-the-board}

1. Check the board first. Let it acclimate to the room where it will be used, and check whether the cup changes. If the wood is still moving as its moisture changes, flattening now may not last.
2. Mark the high areas. Lay a straightedge across the face at several points. Pencil marks on the high areas make it easier to see where material is being removed.
3. Choose the method. Use a plane for a substantial cup if you can control the tool and have enough thickness. Use a long sanding board for mild corrections or when you need to remove material more cautiously.
4. Work gradually. With a plane, take controlled cuts and pay attention to grain direction to reduce tear-out. With sandpaper, keep the backing flat and use even strokes. Recheck across the width and length as you go.
5. Stop when the face is flat enough for the job. Do not keep removing wood just to eliminate every cosmetic mark. Confirm that the finished thickness will still suit the project.

If the board is badly cupped, too thin to flatten safely, or twists as well as cups, a plane or sandpaper alone may not be the right fix. A woodworker can assess whether it can be flattened, or whether the board should be replaced. Once one face is flat, further surfacing the opposite face may require a planer or another suitable setup.