Whether it’s early surface peeling on a recent install or UV-exposed patches appearing years later, flaking topcoat on a GRP fibreglass roof is more than just an aesthetic concern. It can expose the laminate beneath, leading to premature ageing, water ingress, and potential failure of the waterproofing system. Most cases stem from avoidable errors, usually during preparation or curing, and are often made worse by the demands of the UK climate.

In this article, we take a look at the main causes of GRP topcoat flaking, how to assess the damage, what remedial steps are available, and how to avoid the issue altogether in future installations.

What Does a Flaking GRP Roof Look Like?

Flaking GRP topcoat usually manifests as thin or curling fragments that lift away from the surface, leaving behind rough or exposed patches. Affected areas often appear dull, chalky, or faded, with noticeable inconsistencies in sheen. In more severe cases, you may see bare fibreglass matting or resin showing through, particularly around edges or junctions. These symptoms might begin subtly and spread over time, especially if the issue is ignored or worsened by exposure to sunlight and moisture.

Why GRP Topcoats Flake: Common Causes

Topcoat flaking is more than just a cosmetic defect — it’s often the first visible sign of deeper problems in a GRP roofing system. Recognising it early can make the difference between a straightforward re-topcoat and a costly rebuild.

Poor Surface Preparation

One of the most common reasons GRP topcoats begin to flake is inadequate surface preparation. If the topcoat is applied over glossy or unkeyed resin, it struggles to form a proper bond. Even if the surface appears clean, failing to sand it down creates a smooth finish that prevents adhesion. Dust, dirt, and traces of release agents can further interfere with the bond, leading to premature peeling.

Incorrect Curing Conditions

Curing conditions have a significant impact on the long-term performance of a GRP system. In the UK, it is not unusual for roofs to be installed in temperatures that are too low or in damp, changeable weather. Applying a topcoat in temperatures below 5°C or when the humidity is high often results in incomplete curing. A sudden drop in temperature or unexpected rainfall after application can also disrupt the chemical reaction, making the topcoat more fragile and prone to failure.

Damp or Wet Substrate

Curing conditions have a significant impact on the long-term performance of a GRP system. In the UK, it is not unusual for roofs to be installed in temperatures that are too low or in damp, changeable weather. Applying a topcoat in temperatures below 5°C or when the humidity is high often results in incomplete curing. A sudden drop in temperature or unexpected rainfall after application can also disrupt the chemical reaction, making the topcoat more fragile and prone to failure.

How to Fix a Flaking GRP Roof?

Small, localised areas of flaking can usually be repaired quickly by sanding the affected patches with medium-grade abrasive until all loose material is removed. The prepared surface should then be cleaned thoroughly with Acetone or a suitable GRP surface cleaner before applying a fresh layer of topcoat. This approach is often enough to restore both the appearance and weatherproof protection of the roof when the damage is minor.

If the flaking has exposed and caused damage to the main base layer, the repair will require relamination. This process involves sanding down to the laminate, applying new resin to seal and reinforce it, allowing it to cure fully, and then preparing it again before adding the new topcoat. Our GRP Roof Repair Kit has enough material to cover one square metre and contains the resin, catalyst, matting, and topcoat.

Where the deterioration is more extensive, covering large areas or affecting a roof that has been in service for a while, it's often easier to apply our Overcoat System, which is perfect for roofs that have been down a while and need refurbishment or a refresh. Simply clean off the roof, removing any areas of flaking topcoat, then apply the Overcoat system directly to the GRP roof surface. 

How to Prevent Flaking

Prevention relies on correct preparation, accurate mixing and working in the right conditions. Every cured laminate should be sanded before topcoating, even if it looks perfectly clean, with catalyst ratios adjusted to suit the ambient temperature and the substrate completely dry before work begins. One of the most common mistakes is applying the topcoat too thickly to hide the fibre pattern from the laminate beneath, yet this almost always increases the risk of cracking as it cures. The best results come from following the manufacturer’s guidance on application thickness and, if a smoother surface is desired, use Surface Tissue during lamination instead of building up excessive topcoat.