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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.
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) 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.
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.
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.
Air-assist airless systems require additional atomizing air. Keep air-assist air as low as possible during Gelcoat Application.
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.
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.
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.
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 using catalyst injection, ensure the catalyst flows properly. Do not let raw catalyst fall onto the mold or sprayed gelcoat.
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.
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.
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.
K. Overlap strokes by 50 percent.
L. Do not reach with a stroke. Stroke length should be comfortable for the operator, normally 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 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).
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 use. 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. Clean the affected area immediately if contact involves body or clothing. See appropriate data sheets and labels for proper precautionary steps.
U. Know the fire and toxic hazards of polyesters, catalyst, and the particular cleaning solvent being used.
V. Implement a regular preventive maintenance program.
W. Place only one mold in the spray booth at a time to prevent overspray onto other molds.
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.
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 the mold before layup.
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).
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.
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.
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, allowing proper time for equipment cleanup.
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.
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