Adhesives and Sealants: How to Properly Prepare Surfaces for Maximum Adhesion and Long-Term Durability

Photo: Homa Appliances / Unsplash

The success of bonding or sealing depends not only on the quality of the adhesive or sealant itself, but primarily on the proper preparation of the surface. Even the highest-quality adhesive will fail if the surface is not sufficiently clean, dry, and mechanically or chemically treated. In industrial applications, where strength, durability, and long-term joint performance are critical, surface preparation is a key step. Which methods should be chosen for different materials, what mistakes should be avoided, and how can reproducible results be ensured? We answer these questions in a practical guide for manufacturers, technicians, and buyers.

Why Is Surface Preparation Crucial for Adhesive and Sealant Adhesion?

Proper surface preparation is the foundation for achieving maximum adhesion and long-term durability of bonded or sealed joints. Even the highest-quality adhesive or sealant will fail if the surface is not prepared correctly. Adhesion – the bond between the adhesive and the substrate – depends on several factors: chemical compatibility, mechanical anchoring, and surface cleanliness. Dirt, grease, dust, or residues of old coatings create a barrier that prevents direct contact between the adhesive and the material. This leads to joint weakening and premature failure, whether it be peeling, cracking, or loss of sealing.

Another important aspect is surface activation. Some materials, such as polyolefins (e.g., polyethylene or polypropylene), have low surface energy, which means that adhesives do not bond well to them. In such cases, the surface must be treated mechanically (e.g., sanding) or chemically (e.g., plasma treatment, primers). For metals, oxidation plays a role – rust or old coatings must be removed so that the adhesive can form a strong bond with the clean metal. Proper surface preparation is therefore not just a recommendation but an essential step to ensure the reliability and safety of the structure.

How to remove dirt and grease before bonding?

Removing dirt and grease is the first and most important step in surface preparation. Grease, oils, or residues of release agents (e.g., silicones) significantly reduce adhesion because they create a thin layer that prevents direct contact between the adhesive and the substrate. Various cleaning methods are used depending on the type of material and the level of contamination. For metals and plastics, the use of organic solvents such as acetone, isopropyl alcohol, or special hydrocarbon-based degreasers is often recommended. These substances effectively dissolve grease without damaging the underlying material.

For more sensitive materials, such as certain plastics or composites, gentler methods should be chosen. Aqueous cleaning agents with surfactants can be a suitable alternative, but it is important to ensure thorough rinsing and drying of the surface to avoid any residue of the cleaning agent. Mechanical cleaning, such as brushing or sandblasting, is effective for removing coarse dirt, rust, or old coatings. After cleaning, it is advisable to inspect the surface – for example, using a wettability test (a drop of water should spread evenly, not bead up). If the surface is not sufficiently clean, it may be necessary to repeat the cleaning or choose a different method.

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How does surface roughness affect bond strength?

Surface roughness plays a significant role in the mechanical adhesion of adhesives or sealants. A smooth surface provides less area for contact with the adhesive, which can lead to a weaker bond. Conversely, a slightly rough surface increases the contact area and allows the adhesive to better "anchor" into the microscopic irregularities of the material. This is particularly true for materials with low surface energy, where chemical adhesion is limited and mechanical anchoring is key to bond strength.

However, roughness must not be too high, as it could lead to air bubbles being trapped or insufficient adhesive filling. The optimal roughness depends on the type of adhesive and material – for example, fine sanding (sandpaper grit around 120–180) is recommended for epoxy adhesives, while a coarser surface may be more suitable for polyurethane adhesives. Methods such as sanding, sandblasting, or brushing are used to achieve uniform roughness. After surface treatment, it is important to remove dust and impurities that could disrupt adhesion. In some cases, using a primer may be advisable to improve wettability and increase bond strength.

What are the most common mistakes in surface preparation and how to avoid them?

The most common mistake in surface preparation is insufficient cleaning, which leads to weakened adhesion. Many users underestimate the impact of grease, dust, or residues of old coatings that may remain on the surface even after visual inspection. Another mistake is using unsuitable cleaning agents – for example, strong solvents that can damage plastic or composite materials. Similarly, inadequate drying of the surface after cleaning with water-based agents can be problematic, as it leads to moisture being trapped under the adhesive layer and subsequent weakening of the bond.

Another common mistake is incorrect surface roughness adjustment. An overly smooth surface does not provide sufficient mechanical adhesion, while an excessively rough surface can lead to inadequate adhesive filling and the formation of air pockets. It is also important to observe the recommended time intervals between surface preparation and adhesive application – some materials, such as metals, may begin to oxidize or become contaminated again if bonding is delayed too long. To avoid these mistakes, it is advisable to follow the technical data sheets provided by adhesive and sealant manufacturers and, in case of uncertainty, consult an expert.

Photo: Alexandr Popadin / Unsplash

How do humidity and temperature affect surface preparation and adhesive application?

Moisture and temperature are critical factors that can significantly affect the quality of a bonded joint. Excessively high humidity in the air or on the surface can cause water condensation on the material, which disrupts the adhesion of the adhesive. Some adhesives, particularly those based on polyurethanes or cyanoacrylates, react chemically with moisture, which can lead to premature curing or, conversely, insufficient joint strength. The ideal relative humidity during application is typically between 40–60%, although specific values depend on the type of adhesive and material. Before bonding, it is therefore advisable to measure the surface moisture using a hygrometer and, if necessary, dry the surface, for example with warm air or in a drying oven.

Temperature affects both the viscosity of the adhesive and the speed of its curing. Temperatures that are too low slow down chemical reactions, which can prolong the curing time and reduce joint strength. Conversely, high temperatures can cause solvents to evaporate too quickly or accelerate polymerisation, leading to insufficient wetting of the surface. The recommended temperature range for adhesive application is usually between 18–25 °C, but some specialised products require different conditions. It is also important to ensure that the temperature of the surface and the adhesive is balanced – differences can cause thermal stress and weaken the joint. In cold or humid conditions, it is advisable to preheat the surface or use air conditioning in the workspace.

Which surface treatment methods improve adhesion for difficult-to-bond materials?

Some materials, such as polyolefins (polyethylene, polypropylene), fluorinated polymers (PTFE), or silicone rubbers, have low surface energy, making them difficult to bond. To improve adhesion, various surface treatment methods are used to increase wettability and chemical reactivity of the surface. Among the most common are plasma treatment, which uses ionised gas to remove impurities and creates functional groups on the surface that improve bonding with the adhesive. This method is fast, environmentally friendly, and effective even for materials with very low surface energy.

Another effective method is chemical treatment, such as using acidic or alkaline solutions that activate the surface. For polyolefins, etching with chromic acid or potassium permanganate is often used, which increases roughness and chemical reactivity. For industrial applications, corona treatment is also suitable, where the surface is exposed to an electrical discharge, thereby increasing its polarity. For metals, phosphating or chromating is frequently used, which not only improves adhesion but also protects against corrosion. The choice of method depends on the type of material, the required bond strength, and economic considerations – however, it is always crucial to follow recommended procedures and verify the effectiveness of the treatment, for example, with a wettability test or bond strength test.

How to Store and Apply Adhesives and Sealants for Optimal Performance?

Proper storage of adhesives and sealants is essential for maintaining their properties and extending their shelf life. Most products require storage in a dry environment, at a stable temperature, and away from direct sunlight. Some adhesives, particularly those based on solvents or reactive resins, are sensitive to moisture and temperature fluctuations, which can cause premature curing or degradation. For example, cyanoacrylate adhesives should be stored at temperatures below 10 °C to prevent polymerisation. Polyurethane adhesives and sealants, on the other hand, require hermetically sealed containers to avoid reaction with moisture from the air. Before use, it is important to check the expiration date and the appearance of the product – changes in colour, consistency, or odour may indicate a deterioration in quality.

Applying adhesive or sealant requires following several key steps to achieve maximum bond strength. First, it is necessary to thoroughly mix two-component systems according to the manufacturer’s instructions to ensure even activation of the reactive components. For single-component adhesives, it is important to apply the recommended layer thickness – a layer that is too thin may result in insufficient strength, while a layer that is too thick increases the risk of pores or incomplete curing. Application should be carried out in an even layer, ideally using a spatula, gun, or other suitable tool. After application, it is important to observe the open time, during which the bonded parts can be adjusted, and then ensure sufficient pressure during the curing period. For some adhesives, clamps or weights must be used to prevent the bonded parts from shifting during polymerisation.

Need help with selection or application?

Every industrial application requires an individual approach. GCG Group provides detailed technical data sheets and safety data sheets (SDS) for every supplied raw material and advises on the selection of the appropriate adhesive, sealant, or surface preparation method for your specific needs. Contact us – or browse our catalogue of over 1,300 products right away.