Gel Coat Application

How to Properly Apply Gel Coat in Composites

Man putting varnish on wood detail of boat

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Applying gel coat correctly is of utmost importance to making aesthetically pleasing and long-lasting end products. If gel coat is not applied properly it ultimately can increase the cost of the product made, as often is the case, cutting corners in this process will not prove to be worth it.

How Do Improperly Applied Gel Coats Increase Cost?

It depends on a number of parts that get rejected and the work required to fix them. The amount of work and material saved by investing in a proper gel coat application process will pay off in the end. Proper gel coat application includes:

  • Material preparation
  • Equipment calibration
  • Use of trained spray operators
  • Appropriate spray methods

Gel coats should be sprayed and not brushed. The equipment used for spraying must be selected carefully and maintained well.

Catalyst levels are important to the curing of the gel coat and dependent upon shop conditions. Most gel coats’ ideal catalyst level is 1.8 percent at 77°F (25°C), however, specific shop conditions may require this number to vary between 1.2 and 3 percent. Environmental factors that may require an adjustment in catalyst levels are:

  • Temperature
  • Humidity
  • Material age
  • Catalyst brand or type

A catalyst level below 1.2 percent or above 3 percent should not be used because the cure of the gel coated can be affected permanently. Product data sheets can give specific catalyst recommendations.

There are many catalysts for use in resins and gel coats. Proper catalyst selection is vital. In gel coats, only MEKP-based catalysts should be used. The three active ingredients in a MEKP-based catalyst are:

Each component helps the curing of unsaturated polyesters. The following is each chemical’s specific role:

  • Hydrogen peroxide: starts gelation phase, though does little for a cure
  • MEKP monomer: plays roles in initial cure and overall cure
  • MEKP dimer: active during file cure stage of polymerization, high MEKP dimer typically causes porosity (air entrapping) in gel coats

Achieving the correct thickness of a gel coat is imperative as well. A gel coat should be sprayed in three passes for a total wet film thickness of 18 +/- 2 mils thickness. Too thin a coating can result in undercure of the gel coat. Too thick a coat can crack when flexed. Spraying gel coat onto vertical surfaces will not cause sag because of its’ thixotropic characteristics. Gel coats will also not entrap air when applied according to instructions.


With all other factors normal, gel coats are ready for laminating within 45 to 60 minutes after catalyzation. The time is dependent upon:

  • Temperature
  • Humidity
  • Catalyst type
  • Catalyst concentration
  • Air movement

A slowing of gel and cure occurs with low temperatures, low catalyst concentrations, and high humidity. To test whether a gel coat is ready for lamination touch the film at the lowest part of the mold. It is ready if no material transfers. Always monitor equipment and application procedures to ensure proper application and cure of the gel coat.

Material Preparation

Gel coat materials come as complete products and not other materials other than catalysts should be added.

For product consistency, gel coats should be mixed for 10 minutes before use. Agitation should be enough to allow the product to move all the way to the container walls while preventing as much turbulence as possible. It is imperative not to over-mix. This can decrease thixotropy, which increases sag. Overmixing may also result in styrene loss that can add to porosity. Air bubbling for mixing is not advised. It is ineffective and adds for potential water or oil contamination.

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Your Citation
Johnson, Todd. "Gel Coat Application." ThoughtCo, Aug. 28, 2020, Johnson, Todd. (2020, August 28). Gel Coat Application. Retrieved from Johnson, Todd. "Gel Coat Application." ThoughtCo. (accessed June 10, 2023).