Plaster Cornice Repairs: Making Simple Running Molds for Clean Profiles

Running molds are a practical way to reproduce repeated cornice profiles quickly along long runs where the shape is consistent. They give repeatable sections with minimal disruption compared with hand-carving each piece.

They are best for simple, continuous profiles and where the substrate is sound; complex ornament, deep three‑dimensional elements, or structural failure usually need specialist repair or replacement. If unsure whether a running mold is appropriate, check the product label and consider consulting a conservator or structural specialist.

When to repair versus replace

Before you start, assess the damage. Running molds are great for localized repairs.

Repair: If there’s minor damage or a few missing pieces, running molds can restore the profile with minimal disruption.

Replace: If large sections are damaged, consider full replacement. It might be cheaper and less time-consuming than multiple repairs.

Professional help: If there’s structural failure or complex ornamentation, call a specialist. Running molds won’t fix that.

Benefits and trade-offs of DIY running molds

DIY running molds have their pros and cons. Let’s weigh them up.

Cost: DIY is cheaper than buying new or hiring a pro. You just need materials for the mold and repair.

Time: It takes time to make and use running molds, but it’s faster than waiting for custom pieces or professionals.

Finish quality: With practice, DIY can match existing profiles well. But expect some learning curve.

Skill-level: DIY requires some skill and patience. If you’re new to this, start with simpler projects first.

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Start by documenting the cornice condition with photos and notes on material, attachment method, and any patches so you know what you need to match. Look for surface flaws, areas where the cornice has pulled away, and signs of moisture or substrate movement.

Probe gently for hidden problems like rotten lath, crumbling backing, or ongoing leaks that could undermine a repair. If you find possible structural issues, follow up with an appropriate tradesperson and check manufacturer instructions for substrate requirements before proceeding.

Identifying profile types and materials

The cornice’s material and profile shape greatly impact your repair. Using the wrong material can lead to cracking, staining, or weak bonds.

• Plaster: Soft, smooth finish. Look for ‘Gypsum Plaster’ on spec sheets. Avoid using cement-based plasters which are too hard and shrink.
• Gypsum (Drywall): Lightweight, easy to sand. Specs should mention ‘gypsum board’. Don’t use moisture-resistant gypsum for interior repairs.
• Pre-cast: Uniform, factory-made profiles. Check specs for material type and fire rating. Avoid using if not compatible with existing cornice.
• Wood or MDF: Common in older homes. Specs should mention ‘wood’ or ‘MDF’. Don’t use particle board which swells when wet.
• Metal: Rare, usually found in commercial buildings. Specs will list ‘metal cornice’. Avoid using if not compatible with existing attachment method.

Documenting damage and substrate condition

Before making molds, record all damage and substrate issues to ensure a thorough repair. This helps avoid costly surprises later.

Cracks: Note their length, width, and pattern (linear, diagonal, etc.). Hairline cracks may need different treatment than wide ones.

Missing sections: Measure the gap’s size and shape. This helps you create accurate molds.

Substrate condition: Check lath or backing for rot, warping, or missing pieces. Look for moisture damage or signs of pests. If found, address these issues before making repairs to prevent further damage.

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Gather measuring tools (profile gauge or compass, tape), shaping tools (rasps, knives, small planes), and casting items (mixing buckets, spatulas, release agent). Include potential mold materials such as flexible rubbers, foam, or laminated templates and note acceptable substitutes for your skill level.

Also have the plaster or patching compound, reinforcing scrim if needed, and finishing abrasives and fillers ready. Verify compatibility and handling guidance on the product data sheets rather than assuming different materials will work together.

Key properties to check are workability (how easy it is to shape), cure behavior, adhesion to the existing substrate, and flexibility of the mold material to release the profile. Those factors determine whether a plaster mix and a chosen mold material will perform well together during casting and installation.

Don’t assume materials are interchangeable—always read manufacturer guidance for cure times, recommended substrates, and any primer or bonding agent required. If a spec is critical to your project, refer to the product data sheet or contact the supplier for clarification.

Plaster types and key properties

Traditional lime plasters are slow-setting but offer excellent adhesion to old substrates. They’re ideal for historic buildings.

Gypsum plasters set faster, making them convenient for modern projects. However, they may not bond as well to old surfaces without proper preparation.

Modern premixed plasters come in various types. Some are fast-setting, others offer enhanced adhesion or flexibility. Always check the manufacturer’s guidelines to ensure compatibility with your project and existing cornice materials.

Mold materials and flexibility/hardness

The choice of mold material significantly impacts your final cornice profile. Here are key considerations:

• Plaster of Paris: Quick-setting, hardens to a brittle finish. Ideal for intricate, detailed profiles but can crack if not supported properly.
• Gypsum cement: Slower setting, more flexible than Plaster of Paris. Suitable for larger, simpler profiles and offers better resistance to cracking.
• Fiber-reinforced gypsum: Offers enhanced flexibility and strength. Ideal for complex shapes or where the mold needs to withstand multiple castings.
• Urethane rubber: Expensive but provides excellent detail reproduction, flexibility, and durability. Best for intricate profiles and repeated use.
• Silicone rubber: Highly flexible, ideal for complex shapes with undercuts or overhangs. Not suitable for large, flat surfaces due to its thin application.

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Use a profile gauge, wraparound tracing, or rubbed paper to capture the exact contour of the existing cornice and photograph the run for reference. Take measurements at multiple points along the run to catch any slight variations or tapering.

Transfer those recorded lines to a stable template material and double-check fits against the original before making molds. If you encounter inconsistencies, re-measure and adjust rather than guessing to avoid visible mismatches at joins.

Using profile gauges and tracings

To capture the exact curve of your cornice, use a profile gauge or create rubbings. These methods ensure accuracy for seamless joins.

Profile Gauge: Press the gauge firmly against the cornice. Mark the points where it touches. Join the marks to trace the curve.

Rubbing: Place paper or cardboard along the cornice. Trace around it, keeping the paper flat. The resulting rubbing is your template.

Choose from flexible rubber strips for repeated flexible molds, laminated wood or cardboard templates for rigid or one-directional profiles, or one-use foam molds for simple fills and curves. Each method has pros and cons: flexibility and reusability versus ease of shaping and release, so pick what suits the profile and your workload.

Use release agent appropriate to the mold and casting material, and consider making a small trial run before full production to check fit and finish. If you’re unsure which approach to use, consult manufacturer instructions for the chosen mold materials and do a test piece first.

Flexible strip (rubber/silicone) running molds

For long runs, flexible strips are ideal. They allow for continuous casting with minimal seams.

Materials: You’ll need rubber or silicone strips, typically 1/2″ to 1″ wide, and a strong adhesive like contact cement or double-sided tape.

Process: First, measure and cut your strip to match the length of your run. Apply adhesive to both the strip and your template or backing. Press firmly and let it set before casting.

One-use foam or single-use molds

For odd shapes or one-off repairs, disposable foam molds are practical and easy to use.

Materials: You’ll need expanding foam (like Great Stuff), a trigger sprayer for water, and a sharp utility knife.

Process: Spray your template with water to help the foam stick. Apply foam in even layers, letting it expand fully before cutting. Use the utility knife to trim excess foam and create a smooth surface. Let it dry completely before casting.

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Prepare the wall: clean, repair substrate, and apply any bonding agents required by your plaster product, then set up the mold on a jig or straight edge for consistent runs. Mix the plaster to the working consistency recommended by the manufacturer and keep small test batches handy to adjust as you go.

Run the mold steadily, avoiding excessive pressure that squeezes out too much material or under-filled areas that leave voids, and allow proper cure before trimming and joining. Watch joint alignment carefully and follow product cure guidelines rather than assuming a set time.

Preparing the substrate and mold positioning

Before you start casting, prepare the wall surface to ensure your cornice adheres properly. Clean the area thoroughly using a damp cloth or sponge.

Keying is crucial for good adhesion. Use a brush or broom to score the wall, creating tiny grooves that help the plaster grip.

Position the mold carefully along the marked line. Ensure it’s level and straight by using a spirit level. Temporarily secure the mold with painter’s tape or small nails at both ends and in the middle.

Check your work: Double-check that the mold is level, straight, and securely attached before you start casting.

Mixing, applying, and running the mold

The ideal plaster mix should be thick enough to hold its shape but not so thick that it won’t flow into the mold. It should resemble thick peanut butter.

Load the mold by scooping the mixed plaster into one end of the mold using a trowel. Press the plaster in gently to remove air bubbles and ensure full contact with the mold’s surface.

Run the mold smoothly along the wall, applying even pressure. Pull the mold at a steady pace to create a continuous, clean profile. Be careful not to apply too much pressure or under-fill the mold, which can cause voids or an uneven finish.

Preventing voids: To avoid air pockets, press the plaster into the mold firmly and run it smoothly. If you notice any voids, fill them in with more plaster using a trowel.

Curing, removing, and joining sections

Allow the plaster to cure according to the manufacturer’s instructions. This usually takes around 24-48 hours.

Check for curing: The cornice is ready to be removed when it feels firm and no longer gives under gentle pressure. It should also have a consistent, matte appearance.

Remove the mold carefully: Once cured, gently pull the mold away from the wall at a 45-degree angle to avoid damaging the cornice. If any plaster sticks to the mold, scrape it off with a trowel.

Join sections: Butt-join runs together by overlapping them slightly (about 1/8 inch) and applying more plaster to the overlap area using a trowel. This helps create a seamless appearance between sections.

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Fill joints, feather edges, and sand gently to blend the new cornice into the old surface, then reproduce texture as needed before priming and painting. Work progressively and check how light hits the profile to spot any mismatched areas that need further blending.

Create a simple visual checklist: consistent profile line, tight joints, matching surface texture, and even paint finish, and correct any issues before declaring the job done. If paint or texture hides defects, reassess the repair to ensure it won’t show over time and consult product labels for recommended finishing steps.