6 minute read -
Sanding Plasterboard: How to Get Joints and Filled Holes Perfectly Smooth
Only sand plasterboard once the filler is completely dry, using sanding mesh or abrasive paper of P120 to P180, and only on the joints and screw holes. For a few metres a hand sander works fine; for whole walls and ceilings a long-handled drywall sander with dust extraction is much faster and cleaner. Sand lightly, because sanding through the paper facing damages the board.
Sanding plasterboard well is mostly about the right grit, light pressure and good lighting. Here is how to get joints and holes perfectly smooth.
What will you learn in this article?
- Why sand plasterboard?
- By hand or with a machine?
- Which sander should you choose?
- Which grit and which abrasive?
- How to sand plasterboard step by step
- How smooth does plasterboard need to be?
- Safety and dust
- Common mistakes
- Frequently asked questions about sanding plasterboard
- In short: sanding plasterboard
Why sand plasterboard?
After filling the joints and screw holes, you are left with ridges, edges and small bumps. These show through the paint, especially in raking light from a window or lamp. Light sanding levels the transitions between filler and board, so the wall or ceiling looks even once painted.
By hand or with a machine?
| By hand | With a machine | |
|---|---|---|
| Suitable for | A few joints, small repairs | Whole walls, ceilings, new builds |
| Tools | Hand sander with sanding mesh | Drywall sander or orbital sheet sander |
| Speed | Slow | Fast |
| Dust | Lots of dust in the room | Little, with a dust extractor |
| Risk | Low, you can feel what you are doing | Sanding through the paper with too much pressure |
Which sander should you choose?
Drywall sander
A drywall sander has a round, tilting sanding head on a long pole. You can reach the ceiling without scaffolding and the head stays flat against the wall. Most models connect to a dust extractor, so the dust is removed straight away. This is the best choice for large areas.
Orbital sheet sander or hand sander
For a room with just a few joints, you can use an orbital sheet sander with sanding mesh or perforated paper. A rectangular pad sits flat and reaches into internal corners. A cordless sander from the Maxxpack 18V range is handy on a stepladder, as there is no cable in the way. Use a low speed and little pressure.
Random orbit sander
A random orbit sander removes material quickly. On filler that can be too much, leaving dips or bare patches in the paper facing. Only use it with fine sanding mesh and very light pressure.
Which grit and which abrasive?
- P120: for rough edges and bumps.
- P150 to P180: for smoothing the filler.
- P220: for a very smooth surface, for example under gloss paint.
Sanding mesh clogs less quickly than ordinary abrasive paper because the dust falls through the mesh. That is why it is widely used for filler and plaster. Never use a coarse grit such as P60 or P80: it will scratch the paper facing.
How to sand plasterboard step by step
Let the filler dry fully
Wet filler smears and clogs the abrasive. Follow the drying time on the packaging. It depends on the layer thickness, temperature and humidity. Deep holes that you filled in several layers take longer.
Sand only the filler
Sand the joints and screw holes with light pressure. Work from the filler towards the board so the transition becomes smooth. Do not sand the unfilled board, as the paper facing will become rough and fluffy.
Check with raking light
Hold a work light or torch at a low angle along the wall. This shows up small bumps and dips you would otherwise miss. Fill any dips with a thin layer of filler and sand again once dry.
Remove the dust
Brush off the dust with a soft brush or vacuum it with a brush attachment. Dust left on the wall stops primer and paint from adhering properly.
Apply a primer
Plasterboard and filler absorb paint differently. A mist coat or primer makes the paint dry evenly everywhere, so the joints do not show through.
How smooth does plasterboard need to be?
That depends on the finish. Under tiles, the joints only need to be closed. Under wallpaper or textured paint, a level wall without ridges is enough. Under matt emulsion the wall must be flat, and under silk or gloss paint every imperfection will show. In the building trade, finish levels Q1 to Q4 are sometimes used for this, with Q4 being the smoothest.
Safety and dust
Plaster dust is very fine and stays in the air for a long time. Wear a dust mask with at least an FFP2 filter and safety glasses, especially when working overhead. Seal doorways with plastic sheeting and use a dust extractor if you can. An ordinary household vacuum cleaner clogs quickly and can be damaged by fine plaster dust.
Common mistakes
- Sanding before the filler is dry.
- Using too coarse a grit or too much pressure, which damages the paper facing.
- Sanding the whole board instead of just the filler.
- Not using raking light, so imperfections only show after painting.
- Painting without a primer.
Frequently asked questions about sanding plasterboard
Which grit should I use to sand plasterboard?
Start with P120 for ridges and finish with P150 or P180. Only go to P220 for a very smooth, glossy finish.
Can I sand plasterboard with a random orbit sander?
You can, but only with fine sanding mesh, a low speed and very little pressure. The sander removes material quickly and can create dips. A drywall sander or orbital sheet sander is easier to control.
Do I need to sand plasterboard if I am going to wallpaper it?
Yes, the joints need to be level or they will show through the wallpaper. You do not need to work as precisely as for paint. Always prime the wall after sanding with a primer or wallpaper size.
How do I stop dust spreading through the whole house?
Use a sander connected to a dust extractor, seal doorways with plastic sheeting and open a window. Vacuum everything thoroughly afterwards, including floors and windowsills.
In short: sanding plasterboard
Wait until the filler is fully dry, then sand only the joints and holes with P120 to P180 and light pressure. Use a drywall sander for large areas and an orbital sheet sander or hand sander for small jobs. Check your work with raking light, remove the dust, prime before painting and protect yourself from the fine plaster dust.