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Gelcoats are a critical part of most GRP laminates as they enhance both the performance and the appearance of the finished composite. But product quality and consistency alone cannot guarantee trouble-free production.

Good workshop practices are essential and great care must be taken to ensure that mould surfaces are clean and properly prepared before gelcoating. Equally important is the way in which the gelcoat is applied.

Care must be taken to see that the percentage of given colour paste is the same in each gel coat mix. Variations in time and temperature of cure will also affect the consistency of colours between mouldings. It has been found in practice that not only should sufficient mixed colour gelcoat be prepared for all components of a structure at one time, but also that moulding should be laid up and cured under the same conditions of time and temperature.

Certain pigments undergo a change in shade when subject to high temperature. Specially compounded pigment pastes are supplied for use with hot moulding methods and also for resin systems used in cold press moulding where colour can be affected likewise.Following the introduction of isophthalic polyester resins an increase in colour separation has occurred.
Colour separation is primarily the disturbance of the gel coat surface by sag prior to gelation, undetectable during moulding and often caused by resin losing viscosity immediately before gelation.

Gelcoats should, therefore, be applied as evenly as possible and brushed in two directions. Colour separation bears a relationship to high humidity of the atmosphere. The elevation of temperature in a moulding shop often carries with it an increase in humidity causing moisture to condense on cold mould surfaces.

The evaporation of water from release agents will also increase the humidity. Gelcoats of pastel shades which contain a large proportion of white tinting colour develop colour separation more readily since white pigments most commonly used are more sensitive to moisture. Note: Some colours have colour match problems between batches

Check for contaminates such as water or solvent.Dirty equipment.Dry overspray. (Keep a wet line).Excessively applied gelcoat causing sagging.Excessively high delivery rates causing a flooding onto the mould surface.


Establish the correct gelcoat type before starting work

Ensure complete and thorough mould preparation

Stir each pail thoroughly but slowly (to prevent air entrapment) before use

Ensure that gelcoat and moulds are in the temperature range of 16-30°C before starting. Ideally the mould temperature should be 2-3°C higher than that of the gelcoat. Cure will then start on contact giving a more glossy surface.

Keep relative humidity below 80%. High water vapour concentrations in the work area lead to under curing even at elevated temperatures. It is also essential to keep water from condensing on the mould surface.

Ensure that the mould surface is properly treated with release agent. Do not use silicone based release agents. Water based products must be perfectly dry before gelcoat is applied.

Gelcoat is supplied ready for use. Do not add solvents such as acetone. If the application method requires a lower viscosity, up to 2% of styrene may be added.

Catalyse with MEKP at a level of 2%. If catalyst levels are too high or too low, under cure can result making gelcoat less resistant to weathering and water ingress.

If pigment is added, ensure colour fastness and compatibility before use. Add pigment at the recommended level, accurately weighed and mixed in using low shear equipment.

When spraying, build thickness to the required level in 3 or 4 passes to enable fine air bubbles to be released.

If spraying gelcoat ensure the correct nozzle settings and spray pressures and distance are used

Apply an even layer of gelcoat with a thickness from 400 to 600 microns (equal to 550-700 grams per sq.metre). With lower thicknesses gelcoat may be under cured, with higher thicknesses, sags, cracks and porosity can occur. Use a gauge to check correct thickness

Ensure that the mould is well ventilated. Styrene monomer vapours will inhibit the polymerisation and tend to stay in the lower part of the mould because of their higher specific gravity

Apply back-up layers as soon as the gelcoat is sufficiently cured (firm film yet tacky to the touch).


Do not entrap excessive air during mixing of catalyst and pigment

Do not use high shear mixing equipment. This may cause thixotropy breakdown, pigment separation/flocculation, drainage and air entrapment.

Do not dilute gelcoat with solvents other than styrene monomer. When adding styrene do not exceed the maximum level of 2%.

Do not pour gelcoat directly onto the mould before brushing out (this causes shadowing).

Do not use excessive brushing. (Spraying is preferred).

Do not use too rapid a gel time. This does not allow trapped air to escape.

Do not over or under catalyse or pigment.

Do not use silicone based release waxes as these can cause fish eyes.

Recommendations for large mouldings

Because of the high expense involved when gelcoating large mouldings such as boat hulls & decks we recommend using a batch big enough for the job which has been pre mixed by the manufacturer as the pigments are ground directly into the gelcoat during controlled production.

Whichever method is used a small test panel must be produced using the same intended materials including the initial laminate, catalyst levels, mixing process, workshop conditions & operators before commencing with the production mould. This can then be checked for surface faults and a Barcol meter used to check the surface hardness before production begins.

Use of, or reliance on, any information or materials supplied by CFS is entirely at your own risk.

As conditions for use of the materials supplied by CFS are beyond our control, we cannot accept responsibility for any damage or losses for which we shall not be liable. It shall be your own responsibility to check the suitability of any materials used.