Making a potato for moulding: softening and repairing hard lines

A hard arris can make a freshly run cornice look crude, even when the profile is otherwise accurate. It is the sharp ridge or edge left by a running mould, a joint, or an overworked junction. A potato is the plasterer’s small, soft finishing tool for easing that transition and bringing the face back into the flow of the moulding.

The name is trade slang for the tool’s squat, rounded form; the working material can vary. Its real value is control: it can soften a tight line without cutting into the profile. That control depends on making the tool correctly, identifying why the line formed, and matching the repair to the plaster’s stage of set.

Understand the trade tool

In Australian workshops, a potato is commonly improvised from a dense sponge, closed-cell foam, or a suitable foam-rubber offcut. Some plasterers fold the material around a short timber handle, while others work with the rounded pad alone. A natural sponge should be firm enough to retain its shape but soft enough to flex around a cove or ogee. Closed-cell foam gives more consistent control and will not absorb as much water.

Whichever version you use, shape it like a blunt pebble rather than a wedge or square. Pare or sand the edges smooth, round the working face, and check for sharp ridges that could score the plaster. A narrow end can negotiate a tight internal corner, while the broader face can polish a shallow curve. Test the tool on an offcut first, using the same plaster and profile you intend to repair.

A useful test wall makes shape and moisture easier to judge. Mark the profile in millimetres, as Australian plastering suppliers generally work metric, then apply a sample under raking light. When a training room or trade office displays samples, glazing can change glare and colour perception, so office glazing advice is worth checking when designing the display. Keep boards away from direct sun, which can warm the plaster and alter the result.

Find the cause before touching the finish

Before smoothing, decide whether the defect is a proud ridge, a sharp-but-correct arris, a lifted overlap, or a cracked joint. Run a straightedge along the cornice and inspect it under side light; a proud line catches the edge, while a hollow or gap points to poor compaction or movement. Feeling with a clean hand is useful, but a trowel should never be used merely to test the finish.

A dirty seam around a mould joint and a line that continues at a constant height usually begin at the running mould. A line that follows the plaster edge may instead reflect the board or substrate. Any movement, damp staining, or loose fibrous backing needs correction before cosmetic work because a potato cannot stabilise a detached cornice.

Set time is a local reality, not merely a product data-sheet issue. Sydney terraces and Melbourne arcades often contain period fibrous cornices that must be matched sensitively, while humid conditions in Brisbane can keep plaster workable for longer. Hot, dry Perth weather can skin the surface quickly, and Melbourne’s variable conditions make a fixed drying time unreliable. Judge the plaster by its condition rather than by the calendar.

Soften the line at the right moment

Work when the plaster is firm but not fully hard, often called leather-hard. A barely damp potato should glide over the ridge with light pressure. Use controlled strokes that follow the mould’s direction, keeping the tool moving so it does not dwell and gouge. Rinse or squeeze out excess water often; too much moisture brings laitance to the surface and can weaken a delicate edge.

Start at the least visible end where possible and assess by side light before approaching the centre. A thin arris may need several feather-light passes rather than one aggressive rub. If the line remains proud, remove the high spot with a sharp plasterer’s tool and allow the area to firm before using the potato again. The aim is a blended surface, not a uniformly wet shine.

Many hard lines are prevented at source. Keep the mould at a constant angle and speed, clean plaster from its profile between passes, and avoid twisting as it is lifted away. Check fixing screws, shims, and set depth before each run. If you are building a profile for the first time, running mould basics explains the Frenchman, horse, and related setup, making it easier to match the tool to the moulding rather than correct the result afterwards.

Repair a line that has already hardened

A hardened line needs repair rather than rubbing. Cut or scrape away the proud ridge with a plasterer’s knife or scraper, stopping at sound material, then feather the surrounding edge without flattening the profile. Clean the dust and lightly damp the substrate. Fill with a compatible Australian plasterboard finish or moulding compound, slightly proud of the surrounding face, and allow for shrinkage before the final pass.

Where a skim or patch sits on low-suction plasterboard, substrate preparation is critical. Follow the product manufacturer’s instructions and the NAPL bonding-agent guidance rather than brushing on adhesive by habit. Use a compatible primer or bonding system only where specified, and test adhesion on a discreet area. For gypsum-lined walls and ceilings, also check the current AS/NZS 2589 requirements and the product’s technical data sheet.

Build a repeatable repair routine

A reliable routine is simple: diagnose, cut back, fill, firm, soften, and clean. Photograph the defect, note the plaster product, batch, mix ratio, and weather, then record the tool and timing that produced the best result. Those notes are valuable on a large commercial job and when mentoring an apprentice through a TAFE or apprenticeship pathway.

Buy foam, abrasives, timber, and finishing products from reputable Australian suppliers so technical data sheets and safety information remain available. If a crew member or contractor influences purchasing, gifts and rebates should never alter the choice; transparent anti-corruption guidance can support a clear workplace policy. This protects the work, the client relationship, and the credibility of the plasterer.

Quality-check the repair from several angles and with side light, then allow the full finish to cure before decoration or impact. Keep the potato clean, replace it when the sponge becomes loaded with plaster, and store it in a ventilated box so it does not grow mildew. Practise on a scrap cornice section rather than a finished wall, especially when working to a Sydney terrace’s original profile.

Choose one damaged offcut today, build a potato, and practise three controlled passes. Use NAPL’s training material to refine the sequence, then carry the same diagnosis-and-repair method to your next Australian plastering job.