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    Quality assurance in Glass Reinforced Plastic (GRP)

    Quality assurance in Glass Reinforced Plastic (GRP)

    Common Defects

    Numerous grievances related to the visual appeal and functionality of GRP (Glass Reinforced Plastic) mouldings can be attributed to the fundamental issue of insufficient resin curing. Nonetheless, it is essential to carefully examine various problems that occasionally arise in the form of noticeable imperfections or other flaws, as they warrant closer scrutiny. By analyzing the underlying causes, the appropriate remedy for each specific case will become evident.


    This issue arises due to the solvent's impact on the gel coat, caused by the monomer present in the laminating resin, primarily due to the gel coat being under cured. To prevent wrinkling, it is crucial to ensure the correct formulation of the resin, avoid excessively thin gel coats, and maintain control over temperature and humidity. Additionally, it is advisable to keep the work area away from any moving air, particularly warm air. In workshops equipped with hot air blowers, it is recommended to direct them away from the moulds to mitigate this problem.

    Pin holing 

    This problem occurs when the gel coat undergoes solvent attack from the monomer in the laminating resin due to inadequate curing. To prevent wrinkling, it is important to ensure the proper formulation of the resin, maintain an adequate thickness of the gel coat, and carefully control temperature and humidity levels. It is also advisable to keep the work area away from any sources of moving air, particularly warm air. In workshops equipped with hot air blowers, it is recommended to direct the airflow away from the moulds to minimize the likelihood of this issue.


    This defect manifests as small spots scattered across the gel coat surface of the laminate. Typically, it occurs when one of the ingredients in the resin formulation is inadequately dispersed.


    This issue arises from pigment flotation, particularly when using a color mixture composed of multiple pigments. To address this, the recommended solution is to ensure thorough mixing or consider using a different pigment paste.

    Fibre pattern

    The glass fiber reinforcement pattern can occasionally be observed through the gel coat, becoming prominently noticeable on its surface. This commonly happens when the gel coat is insufficiently thick or when the reinforcement is applied and rolled before the gel coat has adequately hardened. Another factor contributing to this issue is premature removal of the molding from the mold. As illustrated below, the panel exhibits a visible fiber print pattern when it catches the light.

    Fish eye

    When working with a meticulously polished mold, especially when utilizing silicone-modified waxes, there is a possibility that the gel coat may "de-wet" from specific areas, resulting in spots where the gel coat is nearly absent. These patches of pale color, typically up to an inch in diameter, become noticeable. Furthermore, this issue can manifest as long, straight lines that follow the brush strokes made during application. However, when a PVAL film is appropriately applied, this fault is seldom encountered.


    The presence of blisters indicates delamination within the molding, where air or solvent has become trapped. Blisters that extend over a significant area may also indicate under-cured resin, and such blisters may only become apparent several months after the molding process. Excessive exposure to radiant heat during curing can also lead to blister formation. One possible cause of this defect is the use of MEKP instead of cyclohexanone peroxide paste. Conversely, if the blister is below the surface, it is likely due to imperfect wetting of the glass fiber by the resin during impregnation. This occurs when insufficient time is allowed for the mat to absorb the resin before rolling. Such blisters of this nature are typically detectable through inspection as soon as the molding is removed from the mold.


    This issue arises when an excessively thick gel coat is applied, and subsequently, the laminate undergoes a reverse impact. It is crucial to ensure that gel coats are never thicker than 0.020 inches (0.5 mm). Additionally, this problem can also be attributed to prolonged exposure to impacts or stress in the affected area over the years.

    Internal dry patches

    Blistering: Blistering may occur if multiple layers of mat are attempted to be impregnated simultaneously. To confirm the presence of internal dry patches, a simple tap on the surface with a coin can be performed.

    Poor Wetting of the Mat: Poor wetting of the mat is caused by either insufficient resin during lay-up or inadequate consolidation. This issue is typically visible only on the reverse face of the laminate without a gel coat. When properly wetted, the laminate should have a glossy appearance due to the resin coating the fibers.

    Leaching: Leaching is a significant fault that arises after exposure of the laminate to weather conditions. It is characterized by resin loss, which leaves the glass fibers vulnerable to moisture damage. Leaching indicates either inadequate resin curing or the use of an unsuitable resin for the intended application.

    Yellowing: GRP laminates tend to develop a slight yellowing effect after prolonged exposure to sunlight. This discoloration can be more noticeable on translucent roof sheeting and white pigmented laminates. Yellowing is primarily a surface phenomenon resulting from the absorption of ultraviolet radiation. Most sheeting resins incorporate UV stabilizers, which significantly reduce the rate of yellowing. It's important to note that yellowing does not affect the mechanical properties of the laminate. Laminates with a higher resin content tend to discolor less rapidly than those with a higher glass content. If a fully cured sheeting laminate with a resin content of at least 75% has its Cellophane protective film removed before exposure to heat or strong sunlight, the degree of yellowing, even after prolonged exposure, will be negligible.

    Repair: Certain molding faults can be easily addressed during the trimming and finishing stage before painting. These repairs should pose no difficulty as all the necessary materials and equipment will be readily available. To begin, remove any loose resin and reinforcement, and clean the affected area with Acetone, ensuring it is thoroughly dried. In some cases, it may be necessary to roughen the surrounding area with abrasive paper to achieve better adhesion.

    For superficial damage limited to the gel coat, apply activated resin or gelcoat to the damaged area and allow it to set. To maintain the resin in position and achieve a smooth finish, applying a layer of Mylar polyester release film is often helpful. It is advisable to apply a slightly thicker film of resin or gelcoat than necessary to account for shrinkage. Once the repaired patch has hardened, dress the resin back to the correct contours of the molding.

    For easier gelcoat repairs, ECF offers comprehensive kits that include all the necessary components. 

    When the damage extends beyond the surface, lay up resin and reinforcement, ensuring the edges overlap to promote good adhesion over a wide area. In cases where the laminate is fractured, remove the entire damaged area and chamfer the inside edge of the hole so that it is larger on the gel coat side than on the reverse side. Roughen the surrounding area to achieve optimal adhesion. If the surface area is large, construct a temporary mold on the exterior surface, applying a release agent and allowing it to dry. For smaller holes, fix a piece of Cellophane over the hole using adhesive tape, which will act as both a mold and a release agent. In cases of extensive damage, it is advisable to replace the molding in its original mold.

    A slightly different repair method can be used for laminates where a strengthening rib can be added. Chamfer the hole to be larger on the inside than on the gel coat side. Carry out the repair as described earlier, ensuring it is no thicker than the laminate. Then, laminate one or more reinforcing ribs over the repaired area, overlapping it as much as possible.

    Inspection: The success or failure of producing high-quality reinforced plastic moldings or laminates depends largely on the fabricator's understanding of the nature of polyester/glass fiber structures and the significance of various stages during fabrication.

    The fundamental distinction between reinforced plastics and most other structural materials lies in the fact that while the chemical composition and properties of materials like steel or aluminum are primarily determined by the manufacturer, the fabricator himself determines these properties in reinforced plastics. In other words, the fabricator creates their own material. As a result, inspection encompasses a broader scope than usual and must be considered at every stage of fabrication. Adequate precautions should be taken to minimize variables and ensure consistency in both materials and fabrication processes.

    During visual inspection of moldings, the following aspects should be carefully scrutinized:

    • Surface imperfections and overall appearance.
    • Absence of air bubbles trapped in the laminate. Visual inspection becomes easier when using unpigmented resin.
    • Dimensions.

    Testing laminates involves certain challenges. It can be broadly categorized into two sections: mechanical testing and chemical testing. The most significant mechanical tests for reinforced plastics include:

    • Ultimate tensile strength.
    • Bend strength (cross-breaking or flexural strength).
    • Modulus in bending.
    • Impact strength.

    However, it is important not to overemphasize high tensile strength values alone. Some types of glass fiber reinforcement may allow the production of laminates with a high ratio of glass to resin. While such laminates may exhibit high tensile strength, they are often too thin to possess sufficient rigidity, rendering them unsatisfactory. Thus, minimum thickness becomes another factor to consider. Furthermore, a polyester/glass fiber laminate with lower resin content will typically have inferior long-term weather resistance.

    Most tests, whether mechanical or chemical, are destructive in nature, meaning that a piece must be extracted from the laminate for the test. It is crucial to cut the sample at least 1 inch away from the edge of the laminate since variations in resin content tend to be more pronounced at the edges. It is also essential to conduct all tests when the laminate has reached maturity and stability. Samples can be post-cured for 3 hours at 80°C to expedite the attainment of stability, but at least 24 hours should pass after gelation before post-curing them.

    Resin-to-glass ratio: Among all the factors, the mechanical and chemical properties of a fully cured laminate are influenced most significantly by the resin-to-glass ratio. Therefore, every GRP workshop should be equipped to conduct this simple test as a routine method of quality control. As a general guideline, a higher glass content results in a stronger laminate, while a higher resin content provides better chemical, water, and weather resistance.

    To determine the resin-to-glass ratio, a small piece of laminate—usually one square centimeter—is weighed in a crucible, ashed over a Bunsen burner, and then reweighed. In cases where a piece cannot be cut directly from the laminate, a test coupon made at the same time as the molding can be used. Although less ideal, a correlation can generally be established between the results obtained from the test coupon and the actual molding.

    Degree of cure: Some variability in properties is attributed to differences in the level of resin cure achieved at the time of testing. Severe under-curing of the laminate is evident as the laminate will be soft and produce a dull sound when tapped with a coin. Detecting slight under-cure is more challenging. Although it may not significantly impact the mechanical properties of the laminate, it will likely lead to rapid deterioration when exposed to the weather.

    A sub-committee of the Joint Services Advisory Board on Plastics has been studying the issue of assessing resin cure since 1953. They have conducted extensive testing programs to find a satisfactory solution. However, as of now, no reliable simple non-destructive test has been identified. Copies of two reports by Sub-committee 2 of the Joint Services Research and Development Committee on Plastics are available upon request.

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