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What is Topcoat?

Put simply, Topcoat (also known as flowcoat) is Gelcoat which has had wax solution added to it, which means it can be used as a finish where it is not possible to use a fibreglass mould, e.g. for GRP Flat roof installations.

Why do we need Topcoat?

All polyester gelcoats suffer from inhibited cure when in contact with the oxygen in the atmosphere. To solve this problem, a small amount (2%) of Wax Solution is added to the gelcoat. The wax is not soluble in gelcoat, and floats to the surface, blocking out the air and ensuring a full surface cure. The resulting Topcoat is used as a finish on external fibreglass surfaces. Topcoat should never be used for anything other than topcoating as resin & gelcoat will not adhere to it because of the added wax.

Polishing and Finishing Topcoat

Topcoat is not a high finish paint, but it does make a very waterproof, hard wearing surface for fibreglass. When dry, some brush marks may be evident, but once it's fully cured it can be sanded with wet & dry paper, and then polished with cutting compound. Click here for our range of Finishing and Polishing equipment.

Preparation of surface

Existing GRP mouldings must be abraded with at least 100 grit sand paper so that the topcoat will adhere. A new fibreglass laminate does not need any preparation. Old fibreglass laminates will benefit from a quick sanding. Painted surfaces usually react badly with topcoat so it is advisable to remove all paint by sanding. Only paint strippers designed for use on fibreglass should be used. All surfaces should be clean & dust free. Finally wipe with acetone solvent and allow to dry.


If you are adding your own wax solution to your gelcoat, it should be added at the rate of 2% by weight. Because of the slippery properties of wax solution, it can take longer for it to be thoroughly mixed, so pay extra attention. Alternatively, you can buy pre-mixed topcoat from CFS.

Adding Pigments

Pigment can be added to the topcoat at a maximum rate of 10% by weight and again, thoroughly stirred. To ensure a uniform colour across the whole job, it's best to add pigment to the total batch of topcoat required to avoid possible shade differences. You can also buy coloured pre-mixed topcoat from CFS in a wide range of colours.

Adding Catalyst

Catalyst must be added and thoroughly stirred in to activate the topcoat. Depending on the temperature of your working conditions, add catalyst at the rate of 2%-3% by weight (but never less than 1.5%). See our Catalyst guide here.

Applying Topcoat

Stir topcoat well before use. You can use your preference of brush or short hair roller, but be sure your tools are resistant to polyester resins. Apply briskly. Once applied, the topcoat will start to skin fairly quickly so avoid fiddling after application. Level the surface as you go.


Approx. 600g of Topcoat per m2.

Topcoat Curing Times

When activated with catalyst at 2% in temperatures of 20ºC, pot life is approximately 20 minutes. Higher temperatures and higher catalyst additions will reduce pot life.

Working in hot conditions

Unfortunately, wax becomes more soluble in hot conditions (particularly direct sunlight) and does not rise to the surface, leading to an inhibited and poorly cured surface area. This under-cured area can remain tacky to the touch and is also prone to soften and swell when exposed to water leading to a white bloom.
If possible, use a temperature sensor to measure the surface temperature of the laminate before applying the topcoat. It is not recommended to apply topcoat on surfaces above 35ºC.
Apply the topcoat out of direct sunlight or wait until the laminate and working conditions have cooled. To gain a good bond to the laminate the topcoat should be applied within 24 hours. If this is not possible the surface should be abraded with 100 grit sand paper, then wiped with acetone before application. Always use low activity catalyst in hot conditions. Stir topcoat well before use to ensure good mixing of topcoat and styrene.

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