Plaster Patch Over Metal Lath: Preventing Rust Bleed and Using the Right Fasteners

Rust bleed is the visible transfer of iron oxidation from the metal lath into the plaster. It happens when moisture mobilizes corrosion products and forces stains into finished surfaces. The result can weaken the bond between plaster and lath and leave stubborn discoloration behind.

Metal lath construction, coatings, and protective primers influence how quickly corrosion progresses. The way the lath is applied and finished can change the chance of rust migrating through plaster. Understanding these variables helps you plan a longer lasting patch.

How Rust Bleed Develops

Rust bleed is a common issue when plastering over metal lath. It occurs due to the chemical reaction between moisture and iron oxide, which causes the rust to migrate through the plaster.

The process starts with moisture penetrating the plaster and reaching the metal lath. When this happens, the iron in the metal begins to oxidize, forming rust. This rust is soluble in water, allowing it to move through the plaster and cause staining or bond failure.

Moisture content in the air, along with the presence of salts or other contaminants, can exacerbate this issue. It’s crucial to understand this process to prevent rust bleed from happening.

Types of Metal Lath and Their Corrosion Resistance

Metal lath comes in various types, each with its own corrosion resistance. Understanding these differences can help you choose the right one for your project.

Expanded metal lath is made from a single sheet of steel that’s stretched and cut to create diamond-shaped openings. It offers good strength but may require additional protection against corrosion.

Woven wire lath is made by weaving thin wires together. Its flexibility makes it easier to install, but its smaller openings can lead to more rust bleed if not properly prepared.

Welded wire lath is similar to woven, but the wires are welded together instead of twisted. This provides better strength and corrosion resistance, making it a good choice for areas prone to moisture or salt exposure. The base metal and any coatings applied also play a significant role in resisting rust, so consider these factors when selecting your lath.

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Begin with a careful visual check for active corrosion and missing lath sections. Look for bright rust, flaky edges, and areas where plaster shows softness or hollow sounds under tapping. Note any staining that may indicate moisture movement.

Determine if surface staining is surface level or if the lath has lost enough support to require replacement. Decide if repairs can be limited to patching or if structural repair is needed before patching.

Replace vs. patch decision criteria

Knowing when to replace or patch lath is crucial. Here are some guidelines:

If you find more than 20% damage, it’s usually best to replace the entire section. Local building codes may require this.

Structural issues, like sagging or bulging plaster, always need professional assessment and likely replacement.

For minor damage, localized patching is often sufficient. But remember, if you’re in doubt, it’s better to replace more than to leave a weak spot that could worsen over time.

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Gather a basic set of hand tools, a stiff brush, and a metal scraper to remove loose rust. Have a rust converter or etch ready if you plan to neutralize corrosion. Include plaster patch material, bonding agents, and compatible fasteners for the lath.

Put on appropriate PPE such as gloves and eye protection, plus drop cloths and clean up supplies. Keep product labels, data sheets, and local code references handy for cross checks during the project.

Check how fastener metals, lath coatings, and plaster types interact to avoid galvanic corrosion. Look for compatibility notes in product data sheets and installation guides. Local rules may influence acceptable materials and methods for exterior or damp interior spaces.

Verify coatings on the lath and recommended plaster mixes before starting. When in doubt, compare multiple sources such as manufacturer instructions and local building code references to guide your choices.

Fastener materials and corrosion compatibility

Choosing the right fasteners is crucial to prevent rust bleed. Material compatibility affects bond strength, longevity, and appearance.

• Galvanized fasteners: Use for exterior or high-moisture areas. Look for ASTM B695 rating. Avoid in highly acidic environments.
• Zinc-coated fasteners: Suitable for interior dry areas. Check ASTM B633 rating. Not recommended for exterior use.
• Stainless steel fasteners: Ideal for all environments, especially where corrosion resistance is critical. Look for ASTM A403/A403M rating. More expensive but long-lasting.
• Aluminum fasteners: Not recommended with metal lath due to galvanic corrosion. Avoid.
• Plastic or composite fasteners: Suitable for interior dry areas where no electrical conductivity is needed. Check manufacturer’s guidelines.

Which plasters bond to metal lath

Metal lath requires specific plaster types for a strong, durable bond. Here are common options:

Bond-coat: A thin layer applied directly onto the lath. It can be gypsum-based or cementitious, depending on the finish coat.

Scratch-coat: Applied over the bond-coat, it provides a key for the final finish coat. Typically, it’s a sanded cement-based or gypsum plaster.

Finish-coat: The final layer that provides the desired aesthetic. It can be gypsum, cementitious, or acrylic-based. Some may require special primers or additives for metal lath application.

Cementitious plasters may require additional reinforcement and expansion control due to their shrinkage properties. Always check manufacturer’s guidelines for specific product requirements.

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Remove loose rust and debris from the lath with a stiff brush and scraper. Stabilize the remaining metal by cleaning it to a bright surface and removing any flaky edges. Apply a rust inhibitor or protective preparation if recommended by the product instructions.

Rinse away dust and let the surface dry before applying any patch materials. Protect surrounding areas and follow label directions for cure times and re-entry guidelines as needed.

Mechanical cleaning and rust removal

Start by removing any loose rust and debris from the metal lath. This is crucial to prevent them from compromising your patch job.

Use a stiff-bristled brush, wire wheel, or sandpaper to scrub away rust. Be careful not to sand through the metal, as this will weaken it. Stop once you reach solid metal.

Note: If you encounter any sharp edges, use a file to smooth them out and prevent injury.

Applying rust inhibitors and primers

After cleaning the lath, apply a rust converter or corrosion-inhibiting primer. These treatments prevent further rust formation and improve adhesion for your plaster patch.

Choose a product that’s compatible with your chosen plaster type and follow the manufacturer’s instructions for application. Typically, you’ll apply it using a brush, ensuring full coverage of the lath surface.

Allow the primer to dry completely before proceeding with your plaster patch. Drying times vary by product, so check the label and plan accordingly.

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Select fasteners that won’t accelerate corrosion and match the lath thickness to avoid deformation. Choose a head type that seats well and drives without bending the metal. Ensure the length reaches solid backing without penetrating too far.

Install with the proper driving technique to avoid stripping or tearing the lath. Check manufacturer guidance and local rules for any special requirements tied to your space or climate.

Fastener selection and head styles

The first step is choosing the right fasteners. You’ll want to match the material of your fasteners with that of your lath to prevent galvanic corrosion.

Common head styles include:

• Bugle heads: These are great for thin materials as they don’t require a large countersink.
• Pan heads: These have a flat top, perfect for situations where you need to drive the fastener flush with the surface.
• Countersunk heads: These are designed to sit below the surface level, ideal when you want a smooth finish.

Always check manufacturer data to ensure compatibility with your lath and environment.

Spacing, length, and installation technique

The spacing of your fasteners is crucial for securing the lath. Typically, you’ll want to space them about 6-8 inches apart, but this can vary depending on the type of lath and substrate.

As for length, it’s important that the fastener penetrates at least 3/4 inch into a solid backing material. If you’re attaching to studs or joists, use fasteners long enough to reach them.

The installation technique is straightforward: align your lath, mark where your fasteners will go, and drive them in using a suitable tool. Be careful not to overdrive the fasteners as this can cause lath deformation.

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Prepare a bond coat that adheres well to both the lath and plaster. Apply a scratch coat with even thickness control and allow appropriate set time per the product instructions. Protect against moisture during curing to prevent stain transfer.

Finish with a top coat that matches the surrounding texture and color, following layering and timing guidelines. Keep an eye on potential rust staining as the plaster cures and adjust technique as needed to minimize bleed.

Applying bond and scratch coats

The bond coat provides a strong base for the final finish, while the scratch coat builds up thickness and improves adhesion. Here’s how to apply them effectively.

Keying into the lath is crucial: Use a trowel to press the plaster firmly into the mesh, ensuring good contact at multiple points.

Control layer thickness: Aim for around 1/8 inch for the bond coat and 1/4 inch for the scratch coat. Too thick can cause cracking or delamination.

Tip: Use a straight edge to check your application depth and ensure even coverage.

Finishing, curing, and painting for protection

Proper curing and finishing ensure the plaster patch integrates seamlessly with the existing surface and provides long-lasting protection.

Curing times vary: Follow the manufacturer’s recommendations for optimal drying and curing. Typically, this takes around 7-28 days.

Sanding and feathering: Lightly sand any high spots or rough edges using fine-grit sandpaper. Feather the edges to blend with the surrounding surface.

Paint for protection: Apply a coat of primer followed by two coats of paint to seal the plaster, block stains, and provide additional moisture protection. Choose a paint designed for use over plaster and compatible with your existing finish.

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Inspect the repaired area for early signs of re-bleed, such as color changes or moisture staining. If rust reappears, reassess the substrate and consider additional stabilizing steps before patching again. Plan for longer term monitoring after repair.

Establish a maintenance routine that includes periodic cleaning and moisture control. Document what worked and what didn’t to guide future repairs and extend the life of the patch.