Drywall tools: names, pictures and what each one is used for

Why there are so many knives, and what each is for

The single most confusing thing about drywall tools is that you appear to need five taping knives that all look the same. You do, and the reason is simple: each coat is wider than the last.

The point of taping is to make a joint invisible, and you cannot do that by filling a narrow line. You do it by spreading the joint so wide and so thin that the eye cannot detect the transition. So the first coat embeds the tape with a narrow knife, the second widens it, and the third feathers the edges out beyond that.

A 150 millimetre knife cannot feather a joint out to 300 millimetres. That is the whole explanation.

Narrow for embedding, wide for feathering

Roughly 125mm to bed the tape, 200mm for the second coat, 250 to 300mm for the third. Each coat wider than the last.

Three thin coats, never one thick one

Thick compound cracks as it shrinks and takes days to dry. Thin coats dry fast and sand to nothing.

Feather the edges, do not build a ridge

The edge of each coat should disappear into the board. A hard edge shows under paint forever, especially in side light.

A trowel is not a taping knife

A flat trowel is for skimming a whole wall. A taping knife is for a joint. They are different shapes for different jobs.

Check the blade is straight

A bent or nicked blade leaves a line in every pass. The blade is the tool, so treat it as a consumable.

Hanging boards, and the mistakes that cause cracks

Cracks in a finished ceiling or wall usually come from how the boards were hung rather than how they were taped. Two things cause most of them: joints landing in the wrong place, and boards fixed too tightly.

Joints should be staggered like brickwork and never line up with the corner of a door or window opening, because that is exactly where movement concentrates. And a board wants to be fixed firmly but not crushed: an overdriven screw tears the paper face, which is the part carrying the strength, so the fixing then does nothing.

Stagger the joints

Never a continuous joint across a ceiling, and never one aligned with a door or window corner. That is where cracks start.

Screw to the depth of a dimple, no further

The head should sit just below the surface without tearing the paper. Torn paper means the screw is holding nothing.

Boards horizontal on walls, perpendicular to joists on ceilings

It puts the long edges, which are tapered and designed to be taped, where the joints need to be.

Leave a small gap at the floor

A board standing in a puddle wicks water up. Ten millimetres off the floor, hidden by the skirting.

Cut tight to the ceiling, loose at the bottom

The top joint is visible and the bottom one is covered. Push the board up tight and let the gap fall where nobody sees it.

Use the right board

Moisture resistant in bathrooms, fire rated where required, and acoustic where sound matters. They are not interchangeable.

The dust, which is worse than people think

Sanding joint compound produces enormous quantities of very fine dust, and it gets everywhere in the building including inside electronics and through closed doors.

There are two answers, and they are both worth using. A sanding system connected to a vacuum captures most of it at source. Wet sanding with a sponge produces none at all, is slower, and is the sensible choice in an occupied house. Either beats a hand sander and an open door, which is how a whole home ends up coated.

Vacuum assisted sanding at source

A pole sander connected to an extractor catches most of the dust before it leaves the wall.

Wet sanding in an occupied house

A damp sponge knocks the ridges back with no airborne dust. Slower, and a great deal less cleaning up.

Seal the room, not just the doorway

Tape the door edges and cover the vents. Compound dust travels through gaps that air barely moves through.

A mask, and the right one

The dust is a fine respirable powder. An FFP3 that actually fits, rather than a paper nuisance mask.

Sand less by finishing better

Every ridge you feather out with a knife is a ridge you do not sand. Good taping is a dust reduction strategy.

What to buy first, in order

Stage one: hanging

A drywall saw, a utility knife with plenty of blades, a rasp, a T square, a tape, a screwgun or drill with a depth setter, and a board lifter.

Stage two: taping and jointing

Taping knives in roughly 125, 200 and 250 millimetres, a mud pan, a corner trowel, jointing tape and compound, and a mixing paddle.

Stage three: sanding and finishing

A pole sander with vacuum connection, sanding sponges, a work light for raking light, and a dust mask that fits.

Stage four: speed and scale

A panel lift for ceilings, a banjo or automatic taper, a corner roller and flusher, and a screwgun with a collated attachment.

Mistakes that show under paint

One thick coat instead of three thin ones

It cracks as it shrinks, takes days to dry, and leaves a mound that no amount of sanding flattens properly.

Not feathering the edges

A hard edge on a coat becomes a visible line under paint, and it is most obvious in light raking along the wall.

Overdriving screws

It tears the paper face, which is what carries the strength. The fixing then holds nothing and the board eventually moves.

Joints aligned with a door corner

The most reliable way to produce a crack. Movement concentrates at openings, so joints must be offset from them.

Skipping the tape

Compound alone in a joint cracks. The tape is the reinforcement, and the compound only holds and hides it.

Sanding into the paper

Once you sand through the face paper it fluffs up and absorbs paint differently, showing as a patch. Seal it or refill it.

Painting without a mist coat

Fresh compound and board face absorb at different rates. Without a diluted first coat, every joint shows through as a shadow.

No light while finishing

Ridges are invisible under overhead light and obvious under a light raking along the wall. Check as you go, not after painting.

Looking after drywall tools

Wash knives immediately

Dried compound on a blade edge leaves a line in every subsequent pass. Thirty seconds under a tap is the whole maintenance.

Never let compound dry in the pan

Set lumps break off into the fresh mud and drag through your finish coat. Wash it out between sessions.

Keep blades straight and unnicked

The blade is the tool. A nick puts a groove in every joint, and it cannot be filed out to true flat.

Do not wash compound down the drain

It settles and sets in the trap. Let it settle in a bucket, pour the water off and bin the sludge.

Store corner tools protected

A corner trowel out of true makes internal corners impossible. It only needs one drop.

Change knife blades often

A drywall utility knife blade blunts within a few boards, and a blunt blade tears the paper instead of cutting it.

Frequently asked questions

What drywall tools do I need to start?

A drywall saw, a utility knife with plenty of blades, a rasp, a T square, a screwgun or drill with a depth setter, taping knives in roughly 125, 200 and 250 millimetres, a mud pan, a corner trowel, tape and compound, and a pole sander.

Why do I need several taping knives?

Because each coat is wider than the last. Taping makes a joint invisible by spreading it so wide and thin that the eye cannot detect the transition, so you bed the tape with a narrow knife then widen and feather with progressively wider ones.

What size taping knives should I buy?

Roughly 125 millimetres to embed the tape, 200 for the second coat and 250 to 300 to feather the third. Three is enough to finish a room properly, and a 150 millimetre knife alone cannot feather a joint wide enough.

What is the difference between a taping knife and a drywall trowel?

A taping knife is a flat blade for working along a joint, coat by coat. A curved drywall trowel is for skimming a whole surface and holds a slight crown that leaves compound in the middle of a joint. They are not substitutes.

How many coats of joint compound do I need?

Three. The first embeds the tape, the second widens the joint, the third feathers the edges out. Three thin coats dry faster, crack less and sand more easily than one thick one, which cracks as it shrinks.

How deep should drywall screws go?

Just below the surface, leaving a shallow dimple, without tearing the paper face. The paper carries the strength, so an overdriven screw that has broken through it is holding nothing at all.

Should drywall be hung horizontally or vertically?

Horizontally on walls in most cases, and perpendicular to the joists on ceilings. That puts the long tapered edges, which are designed to be taped, where the joints fall, and it staggers the joints naturally.

Why does my drywall keep cracking at the same spot?

Almost certainly a joint aligned with the corner of a door or window opening, which is where building movement concentrates. Joints must be offset from openings and staggered like brickwork rather than running continuously.

How do I avoid drywall dust everywhere?

Use a pole sander connected to a vacuum, which catches most of it at source, or wet sand with a damp sponge, which produces none. Seal the door edges and cover the vents, because the dust is fine enough to travel through very small gaps.

Can I use filler instead of joint compound?

Not for joints. General filler is harder, shrinks differently and is very difficult to sand over a wide feathered area. Joint compound is formulated to be spread thin, sanded easily and to move slightly with the board.

Do I have to use tape, or can I just fill the joint?

You have to use tape. The tape is the reinforcement that stops the joint cracking as the building moves, and the compound only bonds it and hides it. A filled joint without tape will crack, usually within the first year.

Why do my joints show through the paint?

Either the edges were not feathered out, so there is a visible ridge, or the wall was painted without a diluted mist coat first. Fresh compound and board paper absorb paint at different rates, which shows as a shadow along every joint.