Conventional Gelcoat application for open moulding

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


Proper application of gelcoat is critical to producing cosmetically appealing and durable parts. Improperly applied gelcoat increases the cost of the part. The amount of additional cost incurred depends on the number of rejected parts as well as the effort required to rework the parts. Making the investment of properly applying the gelcoat can pay big dividends by reducing rework and scrap. Proper gelcoat application includes material preparation, equipment calibration, use of trained spray operators and appropriate spray methods.


A conventional gelcoat is applied with spray equipment. Brushing of gelcoats is not recommended. The following information assumes that the proper gelcoat spray equipment has been selected and that equipment is being properly maintained.

The ideal catalyst level for most gelcoats is 1.8 percent at 77ºF (25ºC). However, the catalyst level can be varied between 1.2 percent and 3 percent to compensate for specific shop conditions. 

Catalyst levels below 1.2 percent or above 3 percent should not be used as the cure of the gel coat can be hindered permanently. Refer to product data sheets for specific catalyst recommendations. There are a number of catalysts available for both resins and gelcoats. It is imperative that the proper catalyst be selected. Only MEKP-based catalysts should be used in gelcoats. Broadly speaking, there are three active components in an MEKP-based catalyst. They are hydrogen peroxide, MEKP monomer, and MEKP dimer. Each of these components play a role in the curing of unsaturated polyesters. Hydrogen peroxide initiates the gellation phase, but does very little for overall cure. MEKP monomer is involved in initial cure as well as overall cure. MEKP dimer is mainly active during the film cure stage of polymerization. Historically, catalysts with high levels of MEKP dimer have been identified as more likely to cause porosity in gelcoats. Environmental factors that may require catalyst range variation include temperature, humidity, material age, and catalyst brand or type. Manufacturers should always verify gel times under specific plant conditions prior to gel coat usage.

The gelcoat should be applied in three passes to a total wet film thickness of 18 ± 2 mils thickness. A coating that is too thin (under 12 mils) could cause undercure of the gelcoat, while a film that is too thick (over 24 mils) could crack under flexing. Gelcoat that is spray applied on vertical surfaces (using this multiple-pass procedure) typically will not sag due to the gelcoat’s thixotropic properties. The gelcoat is also resistant to entrapping air (porosity) when spray applied per instructions.

Under normal conditions, gelcoats are ready for lamination 45 to 60 minutes after catalyzation. The actual time is dependent on temperature, humidity, catalyst type, catalyst concentration, and air movement. Low temperatures, low catalyst concentrations, and high humidity retard gel and cure, meaning that longer times will be required before the gelcoat is ready. A reliable test to determine whether the gelcoat is ready for lamination is to touch the film at the lowest part of the mold. If no material transfers, it is ready for lay-up.

For optimum results, uniform catalyst mix must be achieved. Even with the equipment properly calibrated, problems can occur due to poorly atomized catalyst, surging problems with the gelcoat or catalyst, contamination, and poor application procedures. These problems will quickly negate all benefits of calibration. The equipment and application procedures must be monitored on a routine basis to ensure proper application and cure of the gelcoat. Inquire about and adhere to all equipment manufacturers’ recommendations.


Gelcoat materials are supplied as completely formulated products. No additional materials other than catalyst should be added.

Prior to use, gelcoats should be mixed for 10 minutes to ensure product consistency. The agitation level should allow for product movement all the way to the walls of the container, but with the least amount of turbulence possible. Do not over-mix. Overmixing can break down the thixotropy, increasing the tendency to sag. Overmixing can also cause styrene loss that may contribute to porosity. Do not use air bubbling for mixing. Air bubbling is ineffective and only serves as a potential source for water or oil contamination.

Gelcoats are designed for use at temperatures above 60ºF. Below 60ºF, the viscosity, thixotropy, and cure of the gelcoat are affected.

• The lower the temperature the higher the viscosity.
• The lower the temperature the lower the thixotropy.
• The lower the temperature the longer the gel time.

Catalyst viscosity also increases with decreasing temperatures. This can influence catalyst injector readings.

These factors combine to affect flow rates and atomization as well as make sagging a possibility. In addition, the effect of cold weather on cure can result in poor part cosmetics. The slower gel times and cure times of gelcoats in cold conditionscan lead to postcure that can be seen as print-through and/or distortion.

CCP has developed a few helpful hints to facilitate gelcoat usage during cold weather. These include:

• Calibration of the spray equipment, while always important, is especially important during cold weather due to the increase in viscosity of the gelcoat and catalyst.
• Allow ample time for warming and make sure to check material temperature prior to use. Drums can take two to three days to warm, even inside a warm shop. In extremely cold weather, even longer warming periods may be needed (three to four days). A cold floor will extend the warming time.
• If the plant has a cement floor, there should be insulation (such as a wooden pallet) underneath the material container. This procedure will keep the material warmer by preventing the heat from being drawn out by the concrete.
• Review inventory very carefully and place orders well in advance.


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)

1) Material Delivery Rate, or Fluid Supply—The material delivery rate, or fluid supply, is the rate that 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, allowing maximum material delivery through the gun with the trigger pulled.

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

c) Spray gelcoat into the container for 30 seconds.

d) Reweigh the container and gelcoat in pounds.

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

Adjustment is made by changing the air pressure on the pressure pot or pump, or by changing the orifice size. Material delivery rate checks must be done by weight, not volume, as gel coat densities vary.

2) Atomizing Air — Correct air pressure is essential for proper material atomization. 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 can create erroneous results. Adjust as necessary for a minimum of 60 psi.

3) Catalyst — Proper catalyst level is accomplished by accurate weight or volumetric measurement, so that the catalyst level is exact as well as consistent. 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 can present real fire hazards. Good housekeeping practices need to be maintained at all times.

B. Catalyst Injection — With most catalyst injection equipment, the peroxide catalyst is mixed externally with the gelcoat. 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 have been the direct cause for 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. Also, spraying only catalyst should be avoided.

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

1) Material Delivery Rate or Fluid Supply — calibrate same as for batch mixing.

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.

2) Atomizing Air (Air Volume) — Calibration is the same as described for batch mixing with one exception: the catalyzer has a safety valve that will only allow 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 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 calibration of air pressure needed, or possible. Some airless systems do have air-atomized catalyst, which must be calibrated.

Air-assist airless systems require additional atomizing air. It is important that air-assist air be kept as low as possible.

3) Catalyst — Specific equipment manufacturer’s recommendations should be followed. Calibration methods work as follows:

a) The intent is to collect some catalyzed gelcoat just as it leaves the gun, and time it to see how long it takes to gel. Comparing this gel time to that of a sample that has been catalyzed by accurately weighing the catalyst gives a basis of comparison for adjusting catalyst settings. This should be done at two different catalyst settings. The procedure is to collect about 100 grams of catalyzed gelcoat in a small cup, recording fluid pressure, setting 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 catalyzer by the recommended method specific to the equipment until the two gel times are equal. It should be noted that ball settings are only relative guides and do not read in percent catalyst.

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

4) Do not assume a catalyst slave pump is working properly. These also can be calibrated and should be monitored continually.  CCP’s gelcoat (944-L-A72) contains catalyst indicator which is used to show efficiency of catalyst atomization and mix.


A spray gun is a precision tool. It requires a skilled operator to efficiently apply the material.

Many defects can be traced back to how the gelcoat was applied. A poor spray application can be very costly, so it is in the shop’s best interest to select the proper person as the spray operator and to follow through with good training. 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 there are techniques that must be mastered correctly—from the beginning— to avoid use of bad techniques and costly shortcuts.

New spray operators should start out under direct supervision from competent personnel. They should be assigned to spraying easy, noncritical parts. Progression to more difficult parts should be made in conjunction with the experience and ability of the individual.

Free informational literature is available from suppliers of raw materials and equipment manufacturers. Training schools are offered by most vendors.


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

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

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

D. If catalyst injection is used, make sure catalyst is flowing properly. Do not let raw catalyst fall onto the mold or sprayed gelcoat.

E. Check temperatures; adjust catalyst as necessary (1.2 percent to 3 percent). Under extremely warm conditions, working times may become very short, necessitating the addition of 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.

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. Keep the speed of each stroke so a full and constant wet coat is applied.

J. The first spray pass should be a thin continuous film (5 to 8 mils, dependent upon temperature, gelcoat viscosity, and mold wax). Use of this technique helps to 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 mil gauge in the other.

K. Overlap strokes 50 percent.

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

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.

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

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

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

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

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.

T. Accidental contact with gelcoat or catalyst can be hazardous. In the event of contact involving body or clothing, clean the affected area immediately. See appropriate data sheets and labels for proper precautionary steps to follow.

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

V. Have a regular preventive maintenance program.

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

X. For all around end performance properties, a wet film thickness of 18 ± 2 mils is recommended as ideal. 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 of 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 of great concern, then thinner films of 12 to 16 mils would perform better, but patchability and resistance to print-through and blister could suffer.

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

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

AA. Do not overcatalyze or undercatalyze. Excess catalyst plasticizes gelcoat, thus 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).

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 the outdoor elements is directly dependent on the amount of gelcoat deposited and its quality.

CC. Atomize the gelcoat thoroughly when spraying. Low spray pressures will 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.

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.’

EE. Use the catalyzed gelcoat within its working life with a proper allowance of time for cleanup of equipment.


The shape and contour of each mold will dictate how it can best be gel-coated. This should be considered in planning where to start, where to finish, and how everything in between will be handled. Unfamiliar parts should be given serious consideration as to how they will be sprayed before the actual application begins. Experience will show how it can be done better and more efficiently. Suggestions on spraying different configurations in a mold follow:

A. Try to spray the most difficult area first and work continuously out from it.

B. Keep overspray to a minimum.

C. Use a series of passes perpendicular or diagonal to each other for more uniform thickness.

D. Keep laps (stroke) wet. This is called ‘maintaining a wet line.’ Do not let a lap stay on the mold more than 5 minutes without covering with a ‘fresh’ lap. Alligatoring, and/or resin tearing, and ‘splotches’ could occur when the part is sanded and buffed.

E. Flat areas—These are easy to spray. Begin spraying near an edge in a continuous stroke toward the other side. Each spray pass should be parallel to the previous pass until a uniform thickness is achieved. Subsequent spray passes should be perpendicular or diagonal to the preceding pattern to ensure proper uniform coverage.

F. Corners—Spray a pass down each side through the corner and work out about 12 inches from the corners. Use short strokes, then spray adjacent areas.

G. Gentle Curves—Spray by arcing the gun to keep it perpendicular to the working surface.

H. Channels—Spray the sides first. Most of the time, overspray will cover the bottom.

I. Deep or Narrow Channels—Turn the fluid control in to cut the flow down and narrow the fan. If using airless or air-assist airless equipment, consider a smaller fan. Less fluid and air pressure may be necessary, requiring more passes. Spray the sides first. Do not spray with the fan directly parallel to the channel. Keep the fan perpendicular to the channel (or as much as is possible).

NOTE: If using catalyst injection, cutting back on material flow will change the percent of catalyst supplied to the gelcoat. Adjustment for proper catalyst level will be necessary. Use a 1 quart pot gun to spray very difficult areas.

J. Use a rotating platform for round or small parts.


In general, the brushing of gelcoats is not recommended. Brushing gelcoats will tend to trap air as well as leaving visible brush marks on the part surface. Also, gelcoats are formulated with excess monomer in order to facilitate spraying. Brushing gelcoat will retain this excess monomer in the film.

There are a few instances where brushing gelcoats is either acceptable or unavoidable. An example of an acceptable brushing application is on the backside of a laminate in a non-critical and non-exposed area. This is typically done using an enamel or “air-drying” gelcoat for aesthetic appeal and/or chemical resistance.

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