Fibreglassing over timber boards or solid substrates is one of the most reliable ways to create a seamless, durable and long-lasting waterproof surface. Whether you are working on a flat roof, balcony, pond or walkway, understanding the correct process is essential to achieving a professional finish that stands up to outdoor exposure. You'll therefore need to know how to fibreglass correctly and effectively over plywood, OSB or concrete to meet the needs of your latest project and create a finished surface that won't let you down.

Whether you're a seasoned fibreglass installer or tackling a new project for the first time, getting the preparation and application right makes the difference between a system that lasts decades and one that fails prematurely. Fibreglass, when correctly installed, offers excellent durability, low maintenance and resistance to water ingress, making it a preferred choice across domestic and commercial projects.

However, poor surface preparation, incorrect material selection or a rushed application process can quickly impact the integrity of a fibreglass system. Issues such as delamination, surface cracking and trapped moisture are common when corners are cut, and they often do not become visible until the installation has already begun to fail. In many cases, moisture becomes sealed beneath the laminate, leading to blistering, soft spots, or even structural deterioration over time.

These problems not only affect the visual finish but also compromise the waterproofing performance of the system. As a result, what should have been a long-lasting solution can turn into a cycle of ongoing maintenance and costly repairs that could have been avoided with proper preparation and care.

In this blog, Fibreglass Supplies have assembled a step-by-step guide tailored to plywood, OSB and concrete surfaces, along with practical advice to help you achieve a strong bond and a flawless finish. Whatever your skill level, following these essential guidelines will help ensure faster fibreglassing, consistent results, and long-term performance.

Choosing The Right Substrate For Fibreglassing

Not all surfaces behave the same when fibreglass is applied, and selecting the right base is critical to the success of your project. Plywood is widely regarded as the best substrate due to its stability and smooth finish, while OSB (Oriented Strand Board) can be used with extra preparation. Concrete, although durable, requires careful priming to ensure proper adhesion.

Each material reacts differently to moisture, temperature and resin absorption. For example, plywood provides consistent absorption, whereas OSB can be more porous and uneven. Concrete introduces challenges such as dust, laitance and potential dampness that must be addressed before application.

• Choose exterior-grade or marine plywood with a smooth face. Aim for a minimum thickness of 18mm for roofing applications to prevent flexing over time. Avoid shuttering ply or low-grade boards as they can delaminate and cause failure.
• When using OSB, select OSB3 or OSB4 rated boards. Check that the surface is flat and free from swelling, as uneven areas will show through the fibreglass and weaken the laminate.
• Ensure all boards are fully supported underneath. Install noggins where needed so that no edges are unsupported, which helps prevent movement and cracking later.
• For concrete, check that the surface is fully cured and structurally sound. If the surface is dusty or chalky, it will need mechanical preparation before any fibreglass system is applied.

Preparing Plywood & OSB Surfaces Correctly

Preparation is arguably the most important stage when fibreglassing over timber boards. Even the highest quality resin and matting will fail if the surface is not properly prepared. The aim is to create a clean, dry and stable base that allows the resin to bond effectively.

Plywood and OSB should be securely fixed with appropriate spacing between boards to allow for expansion. All edges and joints must be smooth, and any sharp corners should be rounded off to help the fibreglass mat conform without lifting. Moisture content should be low to prevent future issues.

• Fix boards using corrosion-resistant screws at 150mm centres along edges and 300mm in the field. This prevents movement which can cause cracking in the finished laminate.
• Leave a small expansion gap of around 2mm to 3mm between boards. Do not butt boards tightly together, as natural movement can cause stress cracks in the fibreglass.
• Sand the entire surface lightly, focusing on joints, edges and any rough patches. Wipe down thoroughly afterwards to remove all dust, as dust will weaken adhesion.
• Round off all external corners and edges using sandpaper or a router. Sharp edges can cause the matting to lift or create weak spots in the system.
• Fill any screw holes, gaps or imperfections with a suitable filler and allow it to cure fully before continuing. A smooth, even surface is essential for a professional finish.
• Check that the surface is completely dry before starting. If boards have been exposed to rain, allow adequate drying time or moisture may become trapped under the laminate.

Preparing Concrete For Fibreglass Application

Concrete requires a different approach due to its density and potential for moisture retention. A properly prepared concrete surface can provide an extremely durable base, but neglecting preparation will lead to adhesion problems.

The surface must be completely clean, free from dust, grease and loose particles. Mechanical preparation such as grinding or shot blasting is often necessary to remove laitance and expose a sound surface. Moisture testing is essential, as trapped damp can cause blistering beneath the fibreglass layer.

• Allow new concrete to cure for at least 28 days. Attempting to fibreglass too early can trap moisture and lead to blistering or delamination.
• Mechanically prepare the surface using a grinder or similar tool to remove laitance and create a slightly roughened profile for better adhesion.
• Vacuum thoroughly after preparation to remove all dust and debris. Even fine dust can prevent proper bonding of the resin.
• Check moisture levels using a moisture meter or simple tape test. If condensation forms, the surface is too damp for fibreglassing.
• Repair cracks and holes using an appropriate repair compound. Allow repairs to cure fully and sand smooth before proceeding.
• Apply a compatible concrete primer evenly across the surface. This step is essential to seal the substrate and improve adhesion of the fibreglass system.

Applying Fibreglass Resin & Matting

Once the substrate is prepared, the fibreglassing process can begin. This involves applying a layer of resin, laying the chopped strand mat and saturating it fully to create a strong composite layer. Timing and technique are crucial to achieving a consistent finish.

Work methodically in manageable sections to avoid the resin curing too quickly. Ensure the matting is properly rolled out to remove air bubbles, as trapped air can weaken the structure. Environmental conditions such as temperature and humidity should also be considered during application.

• Pre-cut all fibreglass matting to size before mixing resin. This saves time and prevents mistakes once the resin is catalysed and curing begins.
• Mix resin with the correct catalyst ratio according to temperature guidelines. Too much catalyst can cause rapid curing, while too little can prevent full hardening.
• Apply an even layer of resin onto the surface using a roller or brush. Do not leave dry patches, as these will affect adhesion.
• Lay the matting gently onto the wet resin, starting from one edge and working across to avoid trapping air.
• Use a consolidating roller to press the matting into the resin, working out air bubbles and ensuring full saturation. The mat should turn fully transparent when properly wetted out.
• Overlap joints by at least 50mm to maintain strength and continuity across the surface.
• Work in sections that you can comfortably manage within the resin’s working time, especially in warmer weather.

Finishing With Topcoat For Protection & Appearance

The final stage is applying a topcoat, which seals the fibreglass and provides UV protection along with the desired finish. This layer not only enhances appearance but also plays a critical role in the longevity of the system.

Topcoats are available in a range of colours and finishes, including anti-slip options for walkways and balconies. A properly applied topcoat will protect against weathering, prevent water ingress and ensure the surface remains easy to maintain.

• Allow the laminate to cure fully but apply the topcoat while there is still a slight tackiness if possible. This helps achieve a strong chemical bond between layers.
• Mix the topcoat thoroughly with catalyst, ensuring consistent colour and curing performance across the entire surface.
• Apply using a roller in even, consistent strokes. Avoid overworking the material as this can lead to patchy finishes.
• Check for missed areas or thin spots as you go. These can reduce durability and should be corrected immediately.
• For anti-slip finishes, sprinkle aggregate evenly onto the wet topcoat or mix it in beforehand, depending on the system being used.
• Allow sufficient curing time before exposing the surface to foot traffic or weather. Follow manufacturer guidelines for best results.