Common Problems When Applying Sealant and How to Fix Them

Achieving a trade-quality waterproof finish on your home joints relies entirely on using the right preparation steps, premium sealants, and correct application tools. Many homeowners believe that a successful sealing project only requires grabbing a standard cartridge of hardware silicone and any basic caulking gun. 

However, top quality professional results rely on the preparation that goes into the job before sealant is applied. 

Key Takeaways

• Always strip out old sealants completely and degrease the area because new sealant cannot form a structural bond with cured residues.
• For wide/deep joints, use closed cell PEF backing rod to eliminate three-sided adhesion, allowing the sealant to stretch freely between two substrates.
• Ensure substrates are completely dry before applying the sealant because trapped moisture or solvents may gas out during the curing process.
• Don’t use the soapy finger method for tooling the sealant as this will cause contaminants to be left in the sealant which leads to unsightly black mould growth.

The Reality of Sealant Failure

Sealant failure is rarely a product defect. It is almost always a direct symptom of poor surface preparation, incorrect tool selection, or flawed joint design. When a bead of sealant peels, cracks, or discolours, the root cause can usually be traced back to the installation environment or the application technique.

Understanding the technical reasons behind these failures allows you to troubleshoot issues effectively. By diagnosing the mechanical and chemical forces at play, you can execute a permanent fix rather than a temporary cover-up.

1. The Sealant is Peeling Away (Adhesion Failure)

The Symptoms

The cured bead detaches from the substrate, slips out of the joint easily, or peels away when lightly pulled. This loss of adhesion may be to both surfaces evenly, or to each side of the joint but in different locations, or just to one area of the joint.

The Technical Cause

This issue represents adhesive failure, where the sealant does not form a bond with the surface. This may happen when the surface has contaminants on it, previously applied sealants, moisture in the joint or the surface is designed for things (sealants, dust, dirt) not to stick to it. Furthermore, it could be as simple as the wrong sealant was chosen for the job.

How to Fix It

You must strip the failed joint back completely. Use specialised mechanical scrapers to peel away the bulk material, then apply a chemical sealant remover to dissolve any microscopic plastic films left behind. Clean the joint/gap/area thoroughly with residue-free surface preparation sprays to eliminate contaminants. This may also require the use of specialty ‘adhesion promoters’ that improve the strength of the adhesive bond. This is especially important where the joint is under a stress load (constant water pressure), or used in a critical application (Fire-sealing). Once the substrate is bone-dry and immaculate, reapply a fresh bead of a premium neutral-cure silicone or an advanced hybrid polymer.

Read our guide on removing silicone sealant properly for step-by-step instructions. 

2. The Bead is tearing within itself, and not always in the middle (Cohesive Failure)

The Symptoms

The sealant adheres perfectly to both side walls, but a tear occurs either fully or partially within the surface of the sealant, potentially letting water and drafts penetrate the structure if it has opened up fully.

The Technical Cause

This tear, indicates a cohesive failure within the sealant as a result of several possible events:

1/.  Possible three-sided adhesion within the joint, (both sides and back of the joint),

2/. The sealant has been applied too deep into the joint not allowing correct joint movement when structures shift,

3/.  High structure fluctuations of the sealant while it is curing causing the partially cured sealant to tear, often down the middle and not always all the way through.

4/. The amount of the combined substrate movement is greater than the allowable movement of the joint, usually caused by the panels being too large, or the joint width being insufficient.

How to Fix It

Cut out the damaged split sealant completely back beyond the end of the damaged area by approx. 100mm either side. Before applying new sealant to the joint you may need to insert a PEF Backing Rod, or a PE Bond-Breaker Tape to ensure you maintain the correct joint design and that you do not get three-point adhesion. Ensure that you use the correct ‘adhesion promotor’ for the product and substrate you are working with and allow the correct curing times for the adhesion promoter before applying the new sealant. Apply your fresh sealant over the backing rod, tooling it to guarantee it only bonds to the two opposing side walls. This allows the material to flex and achieve true elastomeric recovery as the building moves.

3. Small Bubbles and Voids Forming Inside the Cured Bead

The Symptoms

Small air pockets, blisters, or hollow voids appear along the surface or within the core of the cured sealant strip.

The Technical Cause

Internal bubbling is caused by a process known as outgassing. This may show up as raised areas on the surface (pimples), or small ruptures in the surface of the sealant. This is more common with polyurethane sealants and almost never seen with silicones and hybrid polymers. There are three main causes of this:

1/. Sealing over a damp joint, trapping the humidity behind the sealant, 

2/. There being too much moisture/humidity in the concrete, along with warm/hot temperatures.

3/. It being hot and humid while the curing is taking place. 

A similar issue may occur if you apply your sealant bead too quickly after cleaning the joint with liquid solvents, trapping the escaping chemical gases under the fresh product.

How to Fix It

Carefully cut out the bubbled sections of the joint. Inspect the internal cavity to verify that no sitting water remains. If you recently used heavy liquid degreasers, wait an adequate timeframe to allow the cleaning solvents to evaporate completely out of the joint pores. Make sure the entire area is totally dry before using a premium caulking gun from our specialised applicator tools selection to re-extrude a slow, steady, continuous bead that pushes out any pocketed air.

4. The Sealant is Turning Black and Mouldy

The Symptoms

Dark black spots, grey streaks, or fungal spores spread rapidly across the surface of the sealant, primarily in high-humidity zones like laundries, kitchens, and showers.

The Technical Cause

While acrylic fillers will grow dark mould spots if not protected by an appropriate paint coating, even technical sealants & silicones with additional fungicide protection can succumb to mould growth in the right conditions. The three most common causes of this are:

1/. Using a dirty smoothing tool to give a nice finish to the sealant. This may include using a wet finger to finish off the sealant.

2/. the joint not being cleaned out properly before the sealant is applied and old mould coming through the sealant from below.

3/. A buildup of contaminants on the surface of the joint that enables the mould to grow down into the sealant.

For mould to grow it requires three things:

1/. contaminants that act as a growing medium and food source, 

2/. Warmth, and 

3/. Moisture

All these things can contribute to counter the anti-mould properties of the sealant.

How to Fix It

Cleaning the surface may provide a temporary cosmetic fix but will not get to the base of much of the fungal growth. Cleaning with bleach will remove any residual anti-fungal properties of the sealant at the surface. You must cut out the mouldy sealant entirely. Disinfect the empty joint cavity with an anti-fungal wash, wash the area with clean water, and let it dry completely. Reapply a premium sanitary silicone. Crucially, abandon the finger method and use profile-shaped rubber or plastic smoothing blocks to compress the joint cleanly.

5. Jagged, Messy Edges Along the Joint Line

The Symptoms

The joint looks uneven, with stringy, jagged fringes of silicone pulling away from the wall onto the tiles or plasterboard.

The Technical Cause

This cosmetic defect occurs because of delayed masking tape removal. Masking tape is an excellent tool for keeping sealant borders crisp, but it must be managed correctly. If you wait until the sealant begins its skin-over time before pulling the tape away, the product will have already initiated its initial cross-linking phase. Pulling the tape tears this semi-solid skin, resulting in an unsealed, ragged edge that easily traps dirt and catches water.

How to Fix It

Clean away the messy, fringed material. When reapplying, lay down fresh masking tape precisely along the borders. Gun your product, tool the bead immediately to its final smooth shape, and then strip the masking tape away while the sealant is completely wet. Pull the tape slowly at a sharp forty-five-degree angle away from the freshly tooled joint line to guarantee a perfectly clean boundary.

Expanding Foam vs Sealant: Choosing the Right Sealant

It is important to select the right category of product for the scale of the gap you are fixing. If you are dealing with a gap wider than 5-7 millimetres, possibly around perimeter floorboards, or pipe work, or may experience moisture, then using a cosmetic sealant is the wrong approach.  Thick layers of acrylic sealant will shrink excessively, slump out of place, or bubble as they struggle to cure in deep spaces. Gaps wider than 5-10 millimetres, you should consider using a hybrid polymer or an appropriate silicone. 

For large voids, you should pivot to expanding foam solutions. Expanding foams are designed to fill deep, irregular cavities, deaden acoustics, and block massive drafts completely. Save your premium elastic sealants for weatherproof joints, visible bathroom lines, and refined perimeter seals. If the expanding foam that you have applied is exposed to moisture or UV, then you will need to coat it with either a sealant or a paint coating to ensure its durability for the medium/long term.

Find Your Solution

Fixing failed caulking requires high-performance formulas, proper preparation, and the right mechanical finishing accessories. Exploring the full Soudal product range ensures you have access to trade-quality solutions that eliminate common joint problems before they start. 

Ready to grab the right supplies to fix a failed bathroom or external window seal? Use our Soudal store locator to find your nearest local hardware or trade stockist today.