Conventional Gelcoat Application for Open Moulding

Conventional Gelcoat Application For Open Moulding

April 14, 2024

INTRODUCTION TO GELCOAT APPLICATION

Proper Gelcoat Application is critical for producing cosmetically appealing and durable parts. Improperly applied gelcoat increases the cost of the part. The additional cost depends on the number of rejected parts and the effort required to rework them. Investing in proper gelcoat application can pay big dividends by reducing rework and scrap. Proper Gelcoat Application involves material preparation, equipment calibration, trained spray operators, and appropriate spray methods.

OVERVIEW OF GELCOAT APPLICATION

Application Method: Use spray equipment for gelcoat application; brushing is not recommended.
Catalyst Levels: The optimal catalyst level for most gelcoats is 1.8% at 77ºF (25ºC). Adjust the catalyst level between 1.2% and 3% to accommodate specific shop conditions. Avoid levels below 1.2% or above 3% to prevent curing issues.
Catalyst Type: Only MEKP-based catalysts should be used. These contain hydrogen peroxide, MEKP monomer, and MEKP dimer, each contributing to the curing process of unsaturated polyesters. Adjustments may be necessary based on environmental factors such as temperature, humidity, material age, and catalyst type.
Application Procedure: Apply the gelcoat in three passes to achieve a total wet film thickness of 18 ± 2 mils. A thickness below 12 mils can result in undercure, while a thickness above 24 mils may lead to cracking under flexing. When applied on vertical surfaces using this method, the gelcoat typically will not sag due to its thixotropic properties and resists air entrapment.
Lamination Readiness: Gelcoats are generally ready for lamination 45 to 60 minutes after catalyzation, depending on temperature, humidity, catalyst type, concentration, and air movement. To determine readiness, touch the film at the lowest part of the mold; if no material transfers, it is ready for lay-up.
Catalyst Mixing: Achieve a uniform catalyst mix for optimal results. Even with properly calibrated equipment, issues such as poorly atomized catalyst, surging gelcoat or catalyst, contamination, and poor application procedures can arise. Regularly monitor equipment and application procedures to ensure proper gelcoat application and cure. Follow all equipment manufacturers’ recommendations.

MATERIAL PREPARATION FOR GELCOAT APPLICATION

Preparation: Gelcoat materials are fully formulated; only add the catalyst. Mix for 10 minutes to ensure consistency, avoiding over-mixing to prevent sagging and porosity. Do not use air bubbling for mixing.
Temperature Considerations: Use gelcoats above 60ºF. Below this, viscosity increases, thixotropy decreases, and gel times lengthen. Cold temperatures also affect catalyst viscosity, flow rates, and atomization, potentially causing sagging and poor cure.
Cold Weather Tips:
- Calibrate spray equipment carefully.
- Warm materials adequately, allowing 2-3 days inside a warm shop, or longer in extreme cold.
- Insulate material containers from cold floors.
- Plan inventory and orders in advance.

EQUIPMENT CALIBRATION FOR GELCOAT APPLICATION

General equipment calibration procedures for material delivery rate and catalyst concentration are discussed below. Always consult the equipment manufacturer for proper calibration of a particular type of equipment.

A. Batch Mix (Hot Pot)

Material Delivery Rate, or Fluid Supply — The material delivery rate, or fluid supply, is the rate at which the gelcoat flows from the spray gun. For optimum spray application, the material delivery rate should be between 1.5 to 2.5 pounds per minute. Determine the material delivery rate as follows:

a) Back out the fluid needle adjustment to allow maximum material delivery through the gun with the trigger pulled.

b) Weigh (in pounds) the container that will capture the gelcoat.

c) Spray gelcoat into the container for 30 seconds.

d) Reweigh the container and gelcoat in pounds.

e) Calculate the material delivery rate in pounds per minute by subtracting the original weight of the container from the container and gelcoat weight, then multiply this figure by 2.

Adjust the air pressure on the pressure pot or pump, or change the orifice size to make adjustments. Perform Gelcoat Application material delivery rate checks by weight, not volume, as gel coat densities vary.

Atomizing Air — Correct air pressure is essential for proper material atomization during Gelcoat Application. To measure, read the pressure gauge attached to the spray gun when the trigger is pulled (dynamic pressure) and the fan is fully open. Adjust as necessary to a minimum of 60 psi. This will help produce a porosity-free film. NOTE: Long air lines, small inside diameter air lines, or a number of fittings within the line can reduce the volume of air supplying the gun and create erroneous results. Adjust as necessary for a minimum of 60 psi.
Catalyst — Achieve the proper catalyst level by accurate weight or volumetric measurement, ensuring the catalyst level is exact and consistent during Gelcoat Application. Always maintain the catalyst level between 1.2 and 3 percent as needed, based on specific plant conditions. NOTE: Catalysts used to cure polyester resins are very reactive chemicals. Contact with many materials can cause decomposition that presents real fire hazards. Maintain good housekeeping practices at all times.

B. Catalyst Injection

With most catalyst injection equipment, the peroxide catalyst mixes externally with the gelcoat during Gelcoat Application. If sprayed alone, it can travel several feet or more, eventually settling onto surrounding surfaces. Accumulation of materials or other substances that can react with the catalyst has directly caused fires in fiberglass shops. Cleanliness and constant removal and proper disposal of waste catalyst and contaminated materials are the only safe ways to deal with this potential hazard. Avoid spraying only catalyst.

Solvent, either from diluting the catalyst as required for some equipment or from cleanup operations, increases the chances of an undesirable reaction. Consult the catalyst supplier, as well as Part Two on ‘Health, Safety, and the Environment’ and CCP Material Safety Data Sheets for further information.

Material Delivery Rate or Fluid Supply
— Calibrate the same as for batch mixing during Gelcoat Application.

a) Air-atomized — 1.5 to 2.5 pounds per minute.

b) Airless — 1.5 to 3 pounds per minute for smaller, intricate molds; 1.5 to 4 pounds per minute for large, open molds.

Atomizing Air (Air Volume) — Calibration is the same as described for batch mixing with one exception: the catalyzer has a safety valve that only allows 80 to 100 psi static air pressure (no air flowing through the gun). The maximum pressure allowed by the safety valve varies with the specific equipment. When maximum static pressure is reached, changing the inside hose diameter, using a shorter hose, and minimizing restrictions will permit more air volume. Airless systems have no air atomization of material, so there is no need or possibility for air pressure calibration. Some airless systems do have air-atomized catalyst, which must be calibrated.

Air-assist airless systems require additional atomizing air. Keep air-assist air as low as possible during Gelcoat Application.

Catalyst
— Follow specific equipment manufacturer’s recommendations. Calibration methods work as follows:

a) The intent is to collect some catalyzed gelcoat just as it leaves the gun during Gelcoat Application and time how long it takes to gel. Comparing this gel time to that of a sample catalyzed by accurately weighing the catalyst provides a basis for adjusting catalyst settings. Perform this at two different catalyst settings. The procedure is to collect about 100 grams of catalyzed gelcoat in a small cup, recording fluid pressure, setting the level of the catalyst ball (or balls), and the time.

Similarly, collect 100 grams that is uncatalyzed, then weigh in the specified amount of catalyst, noting the time of catalyzation. Adjust the catalyzer by the recommended method specific to the equipment until the two gel times are equal. Note that ball settings are only relative guides and do not read in percent catalyst.

b) After calibrating the gel coat (delivery rate), turn off the gel coat. Then run the delivery rate on the catalyst. Compare catalyst delivery to gelcoat delivery (percent catalyst) and adjust the catalyst percent as required to stay in the proper range during Gelcoat Application.

Do not assume a catalyst slave pump is working properly. Calibrate and monitor these continually. CCP’s gelcoat (944-L-A72) contains a catalyst indicator used to show the efficiency of catalyst atomization and mix.

SPRAY OPERATOR FOR GELCOAT APPLICATION

A spray gun is a precision tool, requiring a skilled operator to efficiently apply the material.

Many defects can be traced back to how the gelcoat was applied. Poor spray application can be very costly, so selecting the proper person as the spray operator and providing good training is in the shop’s best interest. A good spray operator should:

• Be conscientious. • Have good coordination. • Desire to do good work. • Have some mechanical skill. • Be patient. • Possess good vision with no color blindness.

Good training is important because the operator must master techniques correctly from the beginning to avoid bad techniques and costly shortcuts.

New spray operators should start under direct supervision from competent personnel. Assign them to spray easy, noncritical parts initially. Progress to more difficult parts should align with the individual’s experience and ability.

Suppliers of raw materials and equipment manufacturers offer free informational literature. Most vendors also offer training schools.

GENERAL SPRAY METHODS FOR GELCOAT APPLICATION

GELCOAT APPLICATION: CHECK FOR CONTAMINATION

A. Check the gun and lines for contamination such as solvent, water, or oil. Clean and correct as necessary before spraying. Drain water from the pressure regulator and traps daily; more often if necessary. If water is a constant problem, leave the bleed-off valve on the water extractor open slightly as a temporary solution. Water in the air lines can lead to expensive repairs to equipment and affect the performance of the gelcoat. Avoid the problem (and reduce costs in the long run) by investing in a good drying system.

GELCOAT APPLICATION: ADJUST AIR PRESSURE

B. Check air pressures before spraying and adjust to achieve proper flow and breakup. Droplets should be no larger than 1/16 inch.

GELCOAT APPLICATION: START SPRAYING NEAR EXHAUST FAN

C. Always start spraying nearest the exhaust fan to minimize overspray that could be pulled onto the mold.

GELCOAT APPLICATION: ENSURE PROPER CATALYST FLOW

D. If using catalyst injection, ensure the catalyst flows properly. Do not let raw catalyst fall onto the mold or sprayed gelcoat.

GELCOAT APPLICATION: ADJUST CATALYST LEVELS

E. Check temperatures and adjust the catalyst as necessary (1.2 percent to 3 percent). Under extremely warm conditions, working times may become very short, necessitating the addition of an inhibitor to allow enough working time. Consult a CCP representative regarding what to add and the amount. Do not go below 1.2 percent catalyst or higher than 3 percent.

GELCOAT APPLICATION: MAINTAIN PROPER SPRAY TECHNIQUE

F. Keep the spray gun perpendicular to the mold during each stroke.

G. Hold the spray gun 18 to 24 inches from the mold when using conventional air-atomized equipment; if using airless equipment, 24 to 36 inches is the proper distance.

H. Do not arc the gun while spraying.

I. Maintain the speed of each stroke to apply a full and constant wet coat.

GELCOAT APPLICATION: APPLY INITIAL SPRAY PASS

J. The first spray pass should be a thin continuous film (5 to 8 mils, dependent upon temperature, gelcoat viscosity, and mold wax). This technique helps prevent porosity, resin tearing, and mottling. About three passes are needed to achieve a total thickness of 18 ± 2 mils. Spraying is a two-handed operation—a spray gun in one hand, and a mil gauge in the other.

GELCoAT APPLICATION: OVERLAP STROKES

K. Overlap strokes by 50 percent.

GELCOAT APPLICATION: MAINTAIN COMFORTABLE STROKE LENGTH

L. Do not reach with a stroke. Stroke length should be comfortable for the operator, normally 18 to 36 inches.

GELCOAT APPLICATION: DEVELOP UNIFORM THICKNESS

M. Begin spraying near an edge in a continuous stroke toward the opposite side. Each pass should be parallel to the former, developing a uniform thickness. Subsequent passes should be perpendicular or diagonal to the preceding pattern to ensure proper uniform coverage.

GELOCAT APPLICATION: SPRAY IN SECTIONS

N. When practical, spray in sections from one end, working continuously to the other. Avoid overspray onto other parts of the mold as much as possible. Do not excessively delay the time lapse between spray passes or in spraying overlapping sections on large molds. Maintain a wet line (i.e., cover up spray edges and overspray as soon as possible).

GELCOAT APPLICTION: AVOID FLOODING GELCOAT

O. Do not flood the gelcoat on or spray with the fan sideways.

GELCOAT APPLICATION: USE MIL GAUGE

P. Use a mil gauge and touch up the tested area afterward.

GELCOAT APPLICATION: CLEAN EQUIPMENT IMMEDIATELY

Q. Clean the gun immediately after use. This includes any part of the equipment that may have received overspray, such as hoses and gauges.

GELCOAT APPLICATION: INSPECT AND MAINTAIN EQUIPMENT

R. Inspect the gun regularly and replace worn parts.

S. Lubricate the gun and packings with light machine oil daily. Do not contaminate the gel coat with oil.

GELCOAT APPLICATION: HANDLE ACCIDENTAL CONTACT

T. Accidental contact with gelcoat or catalyst can be hazardous. Clean the affected area immediately if contact involves body or clothing. See appropriate data sheets and labels for proper precautionary steps.

GELCOAT APPLICATION: UNDERSTAND HAZARDS

U. Know the fire and toxic hazards of polyesters, catalyst, and the particular cleaning solvent being used.

GELCCOAT APPLICATION: IMPLEMENT PREVENTATIVE MAINTENANCE

V. Implement a regular preventive maintenance program.

GELCOAT APPLICATION: LIMIT MOLDS IN SPRAY BOOTH

W. Place only one mold in the spray booth at a time to prevent overspray onto other molds.

GELCOAT APPLICATION: MAINTAIN OPTIMAL FILM THICKNESS

X. For optimal performance properties, a wet film thickness of 18 ± 2 mils is recommended. Films less than 12 mils may not cure properly, may be hard to patch, have more print-through, and be more susceptible to water blisters. Films above 24 mils may prerelease, trap porosity, or crack, and are more subject to weathering discoloration. If water blisters are a great concern (boat hulls), 20 to 24 mils would perform better than a thinner film, but resistance to sag, porosity, and cracking could suffer. If weathering (yellowing from sunlight) is a great concern, then thinner films of 12 to 16 mils would perform better, but patchability and resistance to print-through and blister could suffer.

GELCOAT APPLICATION: AVOID CONVENTIONAL THINNERS

Y. Never reduce gelcoat with a ‘conventional’ paint or lacquer thinner.

GELCOAT APPLICATION: DISPERSE CATALYST THOROUGHLY

Z. Disperse catalyst thoroughly. Poor distribution causes uneven cure, color variation, blister potential, and premature release from the mold before layup.

GELCOAT APPLICATION: MAINTAIN PROPER CATALYST LEVELS

AA. Do not overcatalyze or undercatalyze. Excess catalyst plasticizes gelcoat, degrading its water resistance and accelerating chalking and erosion. Poor cure also results from undercatalyzation. A poorly cured gelcoat is weak and will be degraded by weather. Recommended catalyst level is: 1.2 to 3 percent (1.8 percent at 77ºF (25ºC) ideal) MEKP (9 percent active oxygen).

GELCOAT APPLICATION: APPLY SUFFICIENT GELCOAT

BB. Apply a minimum of 16 mils of gelcoat if glass fiber pattern is to be suppressed appreciably. Never apply less than 12 mils as undercure may take place. The degree of protection against outdoor elements directly depends on the amount of gelcoat deposited and its quality.

GELCOAT APPLICATION: ENSURE THROUGH ATOMISATION

CC. Atomize the gelcoat thoroughly when spraying. Low spray pressures result in poor breakup and leave entrapped air in the gel coat. Entrapped air causes blistering and high water absorption. To check atomization, spray gelcoat over glass to a film thickness of 16 to 20 mils and hold over strong light. Looking through the deposited gelcoat will reveal any entrapped air.

GELCOAT APPLICATION: AVOID WET PVA PARTING FILM

DD. Do not apply gelcoat over wet Polyvinyl Alcohol (PVA) Parting Film. Residual water in the film will retard gelcoat cure and also cause ‘alligatoring.’

GELCOAT APPLICATION: USE GELCOAT WITHIN WORKING LIFE

EE. Use the catalyzed gelcoat within its working life, allowing proper time for equipment cleanup.

SPRAY METHODS FOR PARTICULAR PARTS

Planning: Consider mold shape and contour. Plan the starting and finishing points, especially for unfamiliar parts.
Difficult Areas: Spray the hardest areas first and work outward.
Overspray: Minimize it.
Uniform Thickness: Use perpendicular or diagonal passes.
Wet Laps: Maintain a wet line; cover laps within 5 minutes to avoid defects.
Flat Areas: Spray continuously from edge to edge, using parallel and then perpendicular passes.
Corners: Spray each side through the corner, working out about 12 inches.
Gentle Curves: Arc the gun to stay perpendicular.
Channels: Spray sides first; overspray often covers the bottom.
Deep/Narrow Channels: Reduce flow and narrow the fan. Use smaller fans and lower pressure if needed. Keep the fan as perpendicular as possible.
Catalyst Injection: Adjust catalyst levels if reducing material flow. Use a 1-quart pot gun for difficult areas.
Round/Small Parts: Use a rotating platform.

BRUSHING GELCOATS

In general, avoid brushing gelcoats. Brushing tends to trap air and leave visible brush marks on the part surface. Additionally, gelcoats are formulated with excess monomer to facilitate spraying, and brushing retains this excess monomer in the film.

However, there are instances where brushing gelcoats is either acceptable or unavoidable. For example, brushing is acceptable on the backside of a laminate in a non-critical and non-exposed area. Typically, this is done using an enamel or “air-drying” gelcoat for aesthetic appeal and/or chemical resistance.

Brushing is also occasionally necessary when mold design makes it difficult or impossible to apply a uniform thickness of gelcoat by spraying. In these cases, prespray the area as well as possible. While the gelcoat is still wet, use light, long strokes to brush the overall film to a thickness of 18-22 mils. Alternatively, allow the initial film to gel and brush behind it with catalyzed gel that has been sprayed into a container. In either case, the possibility of alligatoring is relatively high.

Source: CCP Cook Composites and Polymers Co, 2009 Composites Applications Guide, Eleventh Edition.

Frequently Asked Questions

Why is proper gelcoat application important?

Proper gelcoat application ensures a smooth, cosmetically appealing surface and long‑term durability. Poor application leads to defects, rework, scrap, and increased production costs.

What catalyst level should I use?

Most gelcoats perform best at 1.8% MEKP at 77ºF (25ºC). You may adjust between 1.2% and 3% depending on shop conditions. Avoid going outside this range to prevent curing issues.

How thick should gelcoat be applied?

Apply gelcoat in three passes to achieve 18 ± 2 mils wet film thickness.

Below 12 mils → risk of undercure
Above 24 mils → risk of cracking, prerelease, or porosity

How long should I wait before laminating?

Typically 45–60 minutes after catalyzation, depending on temperature, humidity, catalyst levels, and airflow. If the gelcoat no longer transfers to your finger at the lowest mold point, it’s ready.

Can gelcoat be brushed instead of sprayed?

Brushing is not recommended for primary surfaces because it traps air and leaves marks. It’s only suitable for non‑critical, non‑visible areas or when mold geometry prevents spraying.

What temperature should gelcoat be applied at?

Use gelcoat materials at above 60ºF (16ºC). Below this, viscosity increases and cure becomes unpredictable. Always warm materials adequately in cold weather.

What equipment settings are important?

Material delivery rate: 1.5–2.5 lbs/min (air‑atomized), up to 4 lbs/min for large molds

Air pressure: Minimum 60 psi dynamic pressure for proper atomization

Catalyst mixing: Must be uniform for proper cure and color stability

What causes sagging or porosity in gelcoat?

Common causes include low temperatures, poor mixing, incorrect air pressure, improperly calibrated equipment, or spraying too thick in one pass.

How can spray operators reduce defects?

By maintaining proper distance, perpendicular gun angles, consistent stroke speeds, 50% overlap, and by regularly gauging thickness. Proper training and equipment maintenance are critical.

Why is overspray management important?

Overspray can cause defects on other molds, reduce quality, and contaminate equipment. Always spray near the exhaust first and keep only one mold in the booth when possible.