Gelcoat application

The first step to quality moulding. Gelcoat, the pigmented visible layer of a GRP-moulding, has two functions: Cosmetic – color, glaze and surface texture. Technical – protection for the structural laminate.

gelcoat-applicationCorrect catalyst level

Follow the recommendations from the gelcoat supplier for the type and addition of catalyst. A too low catalyst level gives undercuring and to high amount can give a soft result – a rubbery product.

Our machines have mechanically linked polyester and Catalyst pumps for the correct dosage of catalyst. The catalyst is mixed with the gelcoat internally in a static mixer to guarantee a perfect mix.

Working temperature

All lamination work must be carried out at a temperature of at least 18°C. Make sure that all the tools, moulds and materials are conditioned to the actual room temperature. The moisture condensation on a cold mould surface can give serious curing problems. Too cold gelcoat will have a too high viscosity and will be difficult to apply. A 20 kg drum of gelcoat will take at least 48 hours to reach room temperature if taken directly from cold storage.

Gelcoat thickness

Gelcoat should be applied to a thickness of 0.6 – 0.8 mm wet film. Use a wet film guage for control. Avoid runs and excess material in corners and deep sections. A too thin gelcoat film will not cure completely since a high proportion of the styrene will evaporate and there will not be enough left for the cross linking reaction. A too thick gelcoat film can cause cracks, shrinkage and pre-release. An even thickness can be hard to obtain by brush which in addition will give alternate stripes of thick and thin gelcoat. It is much easier to obtain an even gelcoat layer of the correct thickness by spray application. To reduce the styrene loss and be sure of a complete cure one should work with the shortest possible geltime. That will also contribute to a better environment.

Right equipment for quality products

Gelcoat faults are expensive to correct as in most cases the faults can not be detected before demoulding and sometimes not even before the product is delivered. Most of the gelcoat defects are caused by incorrect catalyst dosage and/or bad mixing; these will give an uneven and poor cure. To ensure the right catalyst level the Aplicator machine is designed with a mechanically operated catalyst pump that will give the preset amount independent of variations in air supply or viscosity of material. The catalyst and gelcoat are mixed in an internal static mixer—a standard on all Aplicator gelcoatmachines. Spray application with external mixing of catalyst is in many cases the cause of serious defects such as soft spots, undercured parts, wet patches, dark spots, etc.

External mixing is difficult to control and can be incomplete and involves the risk of catalyst overspray. Futhermore external mixing is a hazard to the environment as it can give a mist of pure peroxide. Curing conditions Let the gelcoat cure in a warm area with adequate ventilation. It must be protected from the sunshine, dust moist and draught.

Keep the machine clean. Keep it clean so you can read the manometers and discover any leakage if they occur. Put a plastic bag over the machine, if anything else will do. Avoid overspray. Test the spray gun in the spray booth into a waste bucket.
Save environment and avoid styrene emission! Even the best ventilating system can not cope with a high background of styrene gas caused by overspray on floors, walls or any place else than the mould. Keep all drums and wessels closed with a tight lid!
Place your moulds in the spray booth in ergonometric position. Tilt and turn the mould instead of climbing on stairs or trying to spray in awkward positions. When adjusting the spray pattern, determine the lowest pressure and the biggest nozzle that gives you the adequate pattern for the work to be done. Remember that high pressure, small nozzles and long distance will increase the styrene emission.
Start the gun outside the moulding area, start on the mould brim. Pull trigger all the way. Halfway open will jeopardise the spray pattern and the work. Spray perpendicular to the surface at a distance of about 50 – 70 cm.

In deep pockets and other parts where it is impossible to maintain the right gun position, take help of a brush.
Be aware that angled spray results in uneven thickness.

Start with wetting the surface. A thin layer, only to wet the mould. Continue with two or three passes to build up the total thickness, should be 0.6-0.8 mm wet film. Check with wet film gauge. Those two or three passes should be sprayed with continuous parallel strokes overlapping 25 % and at constant speed. After finished work, remove the nozzle and flush the gun. Remove the filter from the handle. Clean the nozzle and filter and store them immersed in solvent in a small container with the lid tightly closed.