Repairing cracks in plaster walls is a straightforward DIY project that can revitalise your home's appearance and prevent further damage. This guide will help you understand the process, materials needed, and associated costs for a successful repair.

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Step 1: Assess the Crack

Before you begin, carefully inspect the crack.

• Hairline cracks: These are typically cosmetic and relatively easy to fix. They are often caused by minor settling or drying of the plaster.
• Wider cracks (larger than 5mm): These may indicate more significant issues like structural movement or foundation problems. If you notice cracks that are consistently growing, reappear shortly after repair, or are accompanied by other signs of structural distress (e.g., sticking doors/windows, uneven floors), it's advisable to consult a qualified structural engineer or builder.

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Step 2: Gather Your Tools and Materials

Having all your supplies ready before you start will make the process much smoother. Most of these items are readily available at hardware stores like Builders Warehouse or Leroy Merlin.

• Plaster crack filler or jointing compound: This is the primary material for filling the cracks. You'll find various types, including those specifically designed for plaster or flexible fillers for minor movement.
  • Estimated cost: R80 - R250 for a small tub (e.g., 1kg - 5kg), depending on the brand and type.
• Utility knife or scraper: For widening the crack and removing loose plaster.
  • Estimated cost: R30 - R100.
• Putty knife or broad knife: Essential for applying and smoothing the filler. A set with various sizes can be useful.
  • Estimated cost: R50 - R150 for a single knife, R150 - R300 for a set.
• Sandpaper: Medium-grit (around 100-120 grit) for initial smoothing and fine-grit (around 180-220 grit) for a smooth finish.
  • Estimated cost: R20 - R50 per sheet or pack.
• Dust brush or damp cloth: For cleaning the area before and after sanding.
  • Estimated cost: R10 - R30.
• Primer or bonding liquid: This helps seal the repaired area and improves paint adhesion, especially on porous plaster.
  • Estimated cost: R80 - R200 for a small can (e.g., 1 litre).
• Paint to match the wall: For finishing the repair.
  • Estimated cost: Varies widely depending on brand, colour, and quantity, but expect to pay R150 - R500+ for a 1-litre tin of good quality wall paint.

Total Estimated Material Cost for a Small Repair: R420 - R1480 (This is a broad estimate and can vary significantly based on your choices and the extent of the repair.)

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Step 3: Clean and Prep the Area

This crucial step ensures good adhesion of the filler.

1. Widen the crack: Using your utility knife or scraper, carefully widen the crack slightly into a V-shape. This helps the filler penetrate deeper and create a stronger bond. Remove any loose plaster, dust, or debris from inside and around the crack.
2. Clean the area: Use a dust brush or a damp cloth to thoroughly clean the prepped area. Ensure it's free of any dust, dirt, or grease.
3. Apply bonding liquid/primer (optional but recommended): For best results, especially on older or dusty plaster, apply a coat of bonding liquid or primer to the exposed plaster within and around the crack. This seals the surface and enhances the adhesion of the filler. Allow it to dry completely according to the product's instructions.

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Step 4: Apply the Filler

Now it's time to fill the crack.

1. Load the putty knife: Scoop a generous amount of plaster filler onto your putty knife.
2. Apply the filler: Press the filler firmly into the crack, ensuring it goes deep and fills all voids. Work in small sections, smoothing the filler as you go. Angle your putty knife to feather the edges, blending the filler seamlessly with the surrounding wall.
3. Remove excess: Scrape off any excess filler from the surface of the wall before it dries.
4. Allow to dry: Let the filler dry completely. Drying times will vary depending on the product, the thickness of the application, and local humidity. Always refer to the product's instructions, but it can range from a few hours to overnight. In humid areas like the KwaZulu-Natal coast, drying times might be longer.

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Step 5: Sand and Smooth

Once the filler is completely dry, achieve a smooth, professional finish.

1. Initial sanding: Use medium-grit sandpaper to sand the repaired area. Apply gentle, even pressure, sanding in a circular motion or along the length of the crack. The goal is to remove any high spots and blend the filler with the wall.
2. Fine sanding: Follow up with fine-grit sandpaper to achieve a super smooth finish. This step is essential for an undetectable repair.
3. Clean dust: Thoroughly wipe away all sanding dust with a clean, dry cloth or a damp cloth (ensure the area dries before priming).

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Step 6: Prime and Paint

The final steps to complete your repair.

1. Apply primer: Apply a coat of primer over the entire repaired area. This will seal the filler, prevent "flashing" (where the repaired area looks different from the surrounding paint), and ensure uniform paint absorption. Allow the primer to dry completely.
2. Paint the wall: Once the primer is dry, repaint the wall using paint that matches your existing wall colour. For larger repairs or if you want a seamless finish, consider painting the entire wall from corner to corner. This helps to avoid visible differences in paint sheen or colour. You may need two coats for optimal coverage.

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Final Tips for Success

• For damp areas: If you live in an area prone to dampness (e.g., coastal regions like the Garden Route), consider using a moisture-resistant plaster filler or a specialised damp-proof primer to prevent future issues.
• For recurring cracks: If you experience recurring hairline cracks, a flexible crack filler (often made with acrylic or latex) can be beneficial as it can accommodate minor wall movement.
• Regular checks: Periodically inspect your walls for new cracks or signs of structural movement, especially after significant weather changes. Early detection can prevent larger issues.
• Patience is key: Rushing the drying times between steps can compromise the repair. Always allow sufficient drying time for each product.

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Frequently Asked Questions

How do I know if a crack is serious or just cosmetic?
Hairline cracks (thinner than a credit card) are usually cosmetic. Cracks wider than 5mm, cracks that extend from floor to ceiling or across walls, or cracks that reappear quickly after repair could indicate structural issues. Look for other signs like sticking doors or windows, or uneven floors. If in doubt, consult a structural engineer.

Can I use regular polyfilla for plaster cracks?
While polyfilla can work for very small hairline cracks, it's generally better to use a dedicated plaster crack filler or jointing compound. These products are often formulated for better adhesion, flexibility, and durability on plaster surfaces, making them more suitable for long-lasting repairs.

How long does it take for plaster crack filler to dry?
Drying times vary significantly depending on the product, the thickness of the applied filler, and environmental conditions (temperature and humidity). Always check the product's instructions, but generally, thin applications can dry in a few hours, while thicker applications might take 12-24 hours or even longer in humid South African climates.

Do I really need to prime the repaired area before painting?
Yes, priming is highly recommended. Plaster fillers can be porous, and without primer, the paint might soak in unevenly, leading to a dull or patchy finish. Primer creates a uniform surface, ensures better paint adhesion, and prevents "flashing," where the repaired area looks different from the rest of the wall.

What's the difference between plaster crack filler and jointing compound?
Plaster crack filler is specifically designed for filling cracks in plaster and often has good adhesion and flexibility. Jointing compound (also known as drywall compound or all-purpose filler) is typically used for finishing drywall seams but can also be effective for filling larger plaster cracks, especially if multiple layers are needed. Both can work, but a dedicated crack filler might offer better long-term results for plaster.

How much will it cost me to fix a typical plaster crack DIY?
For a small, typical hairline crack repair, you're looking at material costs ranging from R400 to R1500. This estimate includes basic tools you might not already own (like a putty knife and sandpaper) and materials like filler, primer, and a small tin of paint. The cost can increase if you need more specialised tools, larger quantities of materials, or higher-end products.

Can I fix cracks in painted walls without re-painting the entire wall?
For small, subtle repairs, you might be able to spot-paint the repaired area, especially if you have the exact matching paint and the original paint is relatively fresh. However, for larger repairs, or if the original paint has faded, spot-painting can result in a noticeable patch. For a seamless finish, repainting the entire wall (from corner to corner) is often the best approach.

What kind of sandpaper should I use for smoothing plaster filler?
Start with a medium-grit sandpaper (around 100-120 grit) to remove any major bumps and level the filler with the wall. Then, switch to a finer grit (around 180-220 grit) for a perfectly smooth finish. Always sand gently to avoid damaging the surrounding wall.

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