Sanding drywall: grits, tools and keeping the dust down

Sand less by finishing better

The best sanding job is a short one. Every ridge and lump left by the knife has to come off later, by hand, overhead, in a cloud of dust. Finishers who sand least apply each coat smoothly, knock off ridges with a knife before the next coat rather than sanding between coats, and sand once at the end. Lightweight compound and topping compound sand far more easily than all-purpose and setting compounds; using them for the final coats saves effort.

Grits

Coarser than about 120 cuts the face paper and leaves scratches that show through paint. Sand the compound, not the paper; stop when the ridges are gone and the edges of the joint are invisible to the hand. Fuzzed paper is sealed with primer and lightly re-sanded, or skimmed.

Tools

• Pole sander with a sanding screen or paper: the standard for walls and ceilings, with a swivel head that keeps the pad flat.
• Sanding screens: open-mesh abrasive that does not clog and can be flipped; they last about twice as long as paper on drywall.
• Sanding sponges: for corners, edges and small areas, and wet for dust-free touch-ups.
• Vacuum sanders: a sanding head connected to a shop vacuum, which captures most of the dust at the surface.
• Power sanders with dust extraction: professional tools for large areas, expensive to buy, available to rent.

How many abrasives

There is no published standard for abrasive use; the site’s sanding calculator uses an editorial estimate of about half a sheet of paper per 100 sq ft at Level 4, adjusted for the compound and the tool.

Dust: what it is and how to keep it down

Joint compound dust is very fine. It is mostly calcium carbonate or gypsum, sometimes with talc, mica or perlite; it irritates the eyes, nose, throat and lungs, and OSHA and NIOSH set exposure limits for it as a nuisance dust. It also gets into HVAC systems, electronics and every room of the house.

• Wear a respirator. At least an N95, fitted, and eye protection.
• Close off the room. Plastic sheeting over doorways, with a zip door if you need access; tape over HVAC supply and return registers.
• Turn off the HVAC while sanding, so the system does not spread the dust.
• Use a vacuum sander or wet sponge where you can. A vacuum sander on a shop vacuum with a fine filter captures most dust.
• Clean with a vacuum, not a broom: sweeping puts the fine dust back in the air. A HEPA filter in the vacuum keeps it from blowing through.

Wet sanding

A damp sponge, wrung out until it does not drip, smooths compound by dissolving its surface instead of grinding it into dust. It is slower and does not remove large ridges, but it is almost dust-free and good for touch-ups, patches and occupied houses. Rinse the sponge often; do not soak the joint.

Light is the inspector

Sand with a work light held close to the wall, shining along it rather than at it. The low angle throws shadows from every ridge and hollow, the same raking light that would show them after paint. Circle the flaws with a pencil, fix them, and check again. This is how a Level 4 wall is brought up to the light it will live in.

How long it takes

Sanding is a fraction of the finishing time but most of its mess. The drywall time calculator counts about 5 hours of sanding for 40 sheets at Level 4 by two DIYers, against 20 hours of hanging and 24 hours of finishing. Sanding comes after the last coat is fully dry: the guide to drying times between coats explains how to tell.

Before you sand old walls

Joint compound and textures made before about 1980 may contain asbestos. Have old compound tested before sanding into it; the US EPA’s guidance covers what to do if it is positive. Lead paint is a separate concern on walls painted before 1978; sanding through old paint layers needs lead-safe practices.

After sanding

Vacuum the walls with a brush attachment, or wipe them with a barely damp cloth, so the primer bonds to compound rather than dust. Prime with a drywall primer, which evens the absorption of paper and compound; it also reveals the flaws the light missed, which get a last touch of compound and a light sanding. The finish level decides how much is enough: see the guide to levels of drywall finish. A skim coat, counted by the skim coat calculator, is the fix for walls that sanding cannot save.

Common mistakes

• Coarse grit. It scratches the paper.
• Sanding between every coat. Knock off ridges with the knife instead.
• No respirator. The dust is fine enough to reach the lungs.
• Leaving the HVAC on. The whole house gets the dust.
• Sanding damp compound. It gums the abrasive and tears.