How to Apply Gelcoat – The End-all Guide

Gelcoat PhotoProperly applying gelcoat is the key to a good finish. Many of our customers are hesitant to use it, thinking it’s too complicated or they think they can’t end up with a good finish. So, we are here to provide a step-by-step guide on how to properly apply gelcoat. As with all our products, if you have any questions, we are here to help!

First thing you want to do is look at the surface you want to apply gelcoat to. If the surface is already covered with gelcoat, or if the surface is a fiberglass, or polyester resin, then applying gelcoat will be a snap. If the surface is paint, then the paint would have to be removed before applying the gelcoat.

gelcoat-sandingSo, you’ve decided the surface is gelcoat, so it’s time to begin. You want to sand the surface to create a mechanical bond with the gelcoat. Start by sanding the surface with 150 grit or lower sandpaper. Our technicians recommend using Dyekum Steel Blue to be very thorough. Steel Blue is a dye you wipe on. Once you don’t see the blue dye anymore, you know you have properly sanded everything. You can also use a pencil, but it’s harder to see and won’t guarantee complete coverage.

Next, you want to clean the surface. We recommend using Acetone, as it doesn’t leave a residue and evaporates quickly. Once the surface is clean, you want to get going pretty quickly; if the surface sits for any extended amount of time, you will want to re-clean the surface. Dust and dirt particles are your enemy here, so be thorough on the cleaning.

We are ready to gelcoat! First, decide if you need one or two coats of gelcoat. It will take a minimum of two coats of gelcoat if you are changing colors. If this is going over a patch, we recommend 2 coats at least, to get a nice, uniform surface. Otherwise, one coat will do.

Gelcoat needs to be applied relatively thin. We recommend 18 mils mil-gauge-2thick, to properly cure. 18 mils is approximately the thickness of a matchbook cover. If you aren’t sure how thick, pick up a mil gauge. It’s a quick, easy way to see the thickness of your gelcoat.

Gelcoat requires Methyl Ethyl Ketone Peroxide or MEK-P to activate the hardening process. The amount is very small. We recommend 1-1/4% to 1-1/2% by volume, which means 13-15cc’s per quart. Darker colors require a bit more catalyst for the same reaction, so you can catalyze up to 2%. Refer to the catalyst chart on the side of the can, or you can find one HERE.

Be sure to have a plan of attack for applying your gelcoat. Once you mix it, you have about 15 minutes to apply the gelcoat before it starts to get hard or “gel.” The actual working time depends on your amount of catalyst and how hot your working conditions are. Anything below 60 degrees, and your gelcoat will not cure, but as you get warmer and warmer, your working time decreases rapidly. At 70 degrees, you get your 15 minutes, but at 90 degrees, you only have about 5 minutes. If you need more working time, be sure to sit the can in some cool water, or even some ice to cool it down to 60 degrees give you a bit more time.

Brushable gelFor applying your gelcoat, you can either roll the gelcoat on, like paint, or you can spray it on. We also sell a Brushable Gelcoat that can be applied with a brush. If you decide to roll on the gelcoat, be sure to use a solvent-resistant 1/8” or ¼” nap. Be sure not to use foam rollers, as they tend to leave bubbles. If you brush it on, be sure to use a solvent resistant brush. Your first coat will simply consist of the gelcoat and the MEK-P catalyst. Once applied, you want to wait about an hour and a half, for the gelcoat to set. It won’t be completely cured by then, but it should be hard and tacky.

Your second coat will consist of the same amount of Catalyst, but if you are doing one more coat, your next coat will include Sanding Aid, or ‘wax.’ You will mix in 1 oz per quart to the gelcoat, which will seal the surface from oxygen, causing the gelcoat to dry tack-free. If you roll or brush your seconding coating, be sure to go the opposite direction from the previous coat, allowing a uniform coating.

If you are using our standard Exterior gelcoat, you will be ready to sand and buff your freshly gelcoated surface. If you are using our brushable gelcoat, you will want to let it sit overnight to ensure it’s completely cured. To start sanding, begin with 320 grit sandpaper and sand the surface completely. From then on, start going up to 400, then 600, and finally 800 grit sandpaper.

At this point, you can use an 800 grit compound to compound the gelcoat. You want to use a buffer that turns at 1600-3000 RPM’s. The car polishers will not work, as they spin too slowly, so you want to check the spin speed. It’s best to do a 4’ by 4’ area and go from there. Every 10’ or so, you will want to clean your pad with a spur or with some air to ensure pieces don’t scratch your surface.

If you want a beautiful, glossy shine, use a machine glaze and then two coats of wax, and you will have a beautiful finish that any professional would be envious of.

7 thoughts on “How to Apply Gelcoat – The End-all Guide

  1. The Melvin Grant Company October 13, 2017 / 12:01 am

    I am an architect designing and building geodesic moduled vehicles, but a novice with fiberglassing, so I am gonna need help. Like what’s gelcoat for to start.

    P.s. going to be a frequent customer using PayPal for business purposes

    • FGCI October 13, 2017 / 12:33 pm

      Good day!
      So, gelcoat is used as a sealing, asthetic coat. If you look at a pool, or even a boat, the gelcoat is the color you see. It’s the first coating in a mold. We have two types of gelcoat; the brushable and the standard. You can get gelcoat with wax or without in certain cases. The wax, or Sanding Aid is an additive that allows the gelcoat to dry tack-free. It’s used on the final coat, only. With gelcoat, we recommend at least 2 coats.
      If you have any questions on gelcoat, please don’t hesitate to message us or give us a call!

      FGCI Customer Service

  2. Craig March 5, 2018 / 3:24 pm

    Can gel coat be applied over epoxy resin???

    • FGCI March 5, 2018 / 3:49 pm

      You can’t apply gelcoat directly to Epoxy Resin. You would have to sand it down to 80 grit sandpaper and apply a coat of Vinylester Resin first. At that point, you can apply the gelcoat to that to get a nice finish.

      If you have any other questions, please don’t hesitate to comment, message or call!

      FGCI Customer Service

      • Jacob March 22, 2018 / 4:44 pm

        Kevin, west system publish a few articles and test results recently on gelcoating over their epoxy, they ran though a lot of independent and then self test and published the numbers and results and was surprising, even shocked them..

        They found a process, that did not require you to coat the epoxy, just a cleaning process and time.. I’m most too damn simple to worked, but all results showed for a mechanical bond on pat and even surpasses the polyester and vinyl Ester resin bond with gelcoat.. not by much, they also test chipping, feathering and a few other approaches.. it all started with a customer.. who found a way.. .. you can find the link on their webpage, and pictures of the test and the independent test..

  3. Jacob March 22, 2018 / 4:57 pm

    I did my test gelcoat last evening.. over fgci vinyl Ester resin. Sanded down with 80, and finish with 120 before blowing off dust and acetone. I out mekp at just between 1.75% and 2%, out side in temp was 67 Dec, but part was in Sun a few deg warmer. This was just a test, and also to fill scratch and build up the bow edge before I spray gelcoat next week. (First time working with gelcoat)

    The gelcoat with no wax, brushable, added pigment, let sit for 18 hours.. trying to sand it and it just gunk up my sanding disk each time.. I tried to just take the first layer off then switch to clean pad, still even middle is gunk.. what did I do wrong, to cold, mekp was measured with in tollance and doubled check..

    • FGCI March 22, 2018 / 6:00 pm


      Was the sanding disk cleaned with water before use?

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