Ponds are a great feature to add to gardens and other natural environments. They’re eco-friendly, promote wildlife habitats and increase biodiversity. They’re also a great way to house fish like koi karp.

Fish ponds are typically made from concrete, but this is porous. As such, there is a need for a reliable way to provide waterproofing to protect the concrete structure.

A pond lining is essentially a waterproof coating applied to the inside of a pond. A coating on the other hand would be one applied to the outside of a pond.

Fibreglass (GRP) linings offer seamless, long-term barriers designed to protect substrates like concrete and often last many decades. Versatile and seamless, they can be applied to almost any pond shape or structure.

Why choose fibreglass pond lining over rubber?

As a pond liner, fibreglass (also known as GRP and ‘fiberglass’) is almost identical in price to rubber, but has superior performance relating to longer lifetimes and improved durability and robustness – features of its superior mechanical properties.

Two options:

• Option 1: Choose a fibreglass lining and coating professional to install one
• Option 2: Install the fibreglass (GRP) pond lining yourself

Option 1: Fibreglass pond lining specialists

Strandek has been working with fibreglass and a range of other resins since 1976. You can find out more about our work and experience on our fibreglass linings page.

Option 2: Fibreglass Pond Lining DIY Installation

The following is an installation guide on how to fibreglass a pond.

Fibreglass Pond Liner Installation Guide

The following section details the steps needed to coat the internal surface of a pond with fibreglass.

Resin Library, a supplier of Fibreglass Pond Lining Kits, also offers step-by-step guides and videos.

An instructional video is detailed as follows:

Health and Safety

Health and safety are important factors to consider. Take care to minimise exposure to dust, fumes and sharp objects. Adequate PPE is strongly recommended (dust mask, safety goggles, protective gloves and overalls). Ensure chemicals are kept away from sources of ignition and work is performed in a well-ventilated environment.

Surface Preparation, Fibreglass Laminating & Topcoat Application

Assuming the base structure has been built, several steps are required to create a fibreglass (GRP) pond lining. These are detailed below.

Remove dust, dirt and other contaminating materials and allow the pond surface to dry out. These could be rocks and other objects like leaves and soil.

Note that the re-lining of an existing structure should remove the existing lining. Depending on the type, some degree of scaling back may be required in order to expose the underlying concrete. This is largely detailed in the next section.

Ponds are generally constructed from concrete and mortar or an equivalent. Concrete typically necessitates some mild scaling to ensure a flat surface free from sharp edges.

Surface preparation: scale back a thin layer of the external concrete to create a consistent surface. The purpose here is to remove sharp edges and regions – particularly corner edges. Use a sandblasting or an equivalent, such as a mechanical grinder. A slight texture to the concrete surface is desired as it will improve the adhesion between the fibreglass laminate.

Clean the pond surface: surface preparation will have generated a degree of dust and dirt. Remove all loose debris and dirt before proceeding.

Dry the concrete surface out to an acceptable level. The measurement of concrete moisture has been described elsewhere.

Note: it is always wise to perform fibreglassing work under dry conditions, so be sure to check the weather forecast in advance and plan accordingly. Should unplanned rainfall occur, plastic sheeting can be applied to keep the pond dry.

Glass fibre matting preparation: in advance of any resin preparation, glass fibre matting (typically 600-gram CSM) should be sized and cut in advance into appropriate sections.

Note: a 10% excess should be targeted per sheet to provide sufficient overlap (usually 5 cm) between sections. This is important to achieve a seamless finish.

Resin primers are recommended to provide an extra layer of concrete protection from the leaching of external contaminants. A thin layer should be created with the amount depending on the porosity of the concrete. Approximately 0.25 to 0.5 kg per m2 is a good target. Allow the primer to cure for 30 – 60 minutes such that the finish is slightly tacky/sticky.

Polyester laminating resin: prepare workable amounts of polyester resin by adding MEKP catalyst in a 1 to 4 % volume per weight ratio. E.g., for every 1 kg of polyester resin, add and carefully mix 10 mL of MEKP.

The amount of resin per square meter of glass fibre depends on the glass fibre density. If 450 gram CSM is being used, apply 1.2 – 1.4 kg per square meter. If 600 gram CSM, apply 1.5 – 1.7 mg per square meter.

Apply the catalysed polyester resin to the glass fibre matting promptly using a polyester roller. Consolidate the resin laminate using a paddle roller – this is important to drive out air bubbles and to minimise unsightly creases to create a consistent surface.

Once a consistent finish to the laminate has been achieved, allow it to cure until mildly tacky. This can take up to 24 hours in colder conditions. If more than 24 hours goes by due to unforeseen issues, clean the laminate with acetone wipe to remove dust and any dirt.

Inspect the laminate to ensure consistency and quality. If glass fibres are exposed, sand back and re-apply resin as appropriate.

Resin topcoat: in the same manner as polyester laminating resin, prepare the topcoat. This also requires 1 – 4 % volume per weight of MEKP catalyst to be added and carefully mixed. Once prepared, apply approximately 0.5 kg per square meter of laminate using a polyester roller. Ensure that the laminate is completely covered, taking to care to achieve a thin, consistent layer of coverage.

Allow the resin topcoat to cure. Again, this can take up to 24 hours and the final finish should cure to form a dry, solid barrier. Prior to introducing fish (like koi) and other aquatic life., rinse the surface of the pond with water and leave overnight prior to draining. Repeat as desired.

Frequently Asked Questions (FAQs)

Q. What type of fish can be introduced?

A. Any type that the pond environment can tolerate. Classic examples include koi and goldfish, as well as many other pond fish types.

Q. How can I design a pond?

A. Guidance on pond design has been outlined by Freshwater Habitats. Please see the guide above for more information on fiberglass ponds.

Q. Can I use fibreglass to make a water feature or a preformed pond?

A. Yes, fibreglass moulds can be created for a variety of structures.

Q. Can rubber liners be used to make a GRP koi pond?

A. Rubber pond liners like EPDM are used for ponds, but have a shorter lifetime. They are also susceptible to tearing and puncturing due to limited mechanical properties. Rubber liners also accumulate air pockets underneath the pond, increasing the likelihood of a tear and leak.

Q. Do ponds increase biodiversity?

A. Yes, a garden pond is an excellent way to increase biodiversity. They also have many biodiversity benefits in urban environments.

Q. Why do I need a fibreglass coating for my pond?

A. Fibreglass linings refer to the internal surface coatings of ponds. Substrates used to construct ponds (namely concrete) are porous. As such, groundwater can penetrate the concrete and seep out of the pond – this is particularly common when repeated freeze-thaw cycles occur. Furthermore, leaching of concrete ingredients and pollutants from the surrounding soil into the pond water are toxic to fish. Fibreglass linings applied to concrete form an excellent barrier between the concrete and water – furthermore, when applied correctly they last decades.

Q. Do I need a pond filter for my fish?

A. No. Provided there are aren’t too many fish in the pond a natural equilibration can be achieved by introducing aquatic plants into the pond. These act as natural filters for the fish.

Q. Can a GRP liner also be used for swimming pools?

A. Yes, the same principle of installation applies to swimming pools.

Q. What does the presence of algae indicate?

A. Too much nitrogen and phosphorus can lead to algae. In a pond – or even a lake – algae will consume oxygen and block sunlight from underwater plants. The best way to eliminate algae is to add plants, although other methods are also cited for algae control.

Q. What other pond GRP lining systems are available?

A. As mentioned, rubber lining systems are used – these are a less robust alternative to fibreglass – although they are a form of flexible pond liner – and pose problems in the event of a puncture. With fibreglass, the final layer of resin creates a perfect seal that is watertight and durable.