Don’t Make These Mistakes when Fixing Cracks and Chinks in Fiberglass

Fixing cracks, scratches, and chinks in fiberglass is a pretty easy process which often doesn’t work out too well. It should be simple, right? Just mix the resin and hardener, add a little pigment, and smear it on. Well… sort of. If you have something you need to fix on your boat, give this a quick read-through before you get started – it could make the difference between a clean looking repair job, and making your boat look worse than it did when you got started. 

One big mistake people make is doing the repair in the evening, after work. This is a bad move because as the sun goes down the temperature changes rapidly; that changes the rate at which the gel coat hardens. You can end up with a dry outside but a gooey inside, which never really sets properly.

Another bad move is adding the hardener to the gel coat before mixing in the pigment. It can take a few tries to get the color match right, and if you add the hardener first, it’ll become tougher and tougher to thoroughly mix while you add the pigment.

Botching the job also happens when people put on the gel coat, and leave it completely exposed to the air. Gel coat hardens more uniformly if you cover the repair with plastic wrap or a similar film, as it dries.

Another gel coat no-no is painting it on with a brush. Sometimes this works out okay, but other times the brush strokes remain in the finished product. Generally speaking, a plastic resin spreader works best.

cracks in gel coat

As you can see, a sloppy gel coat repair job can make your boat worse than it did before you got started.

When filling cracks in a horizontal surface, never walk away from the job before it has a chance to soak in. Deck cracks, in particular, may swallow up  gel coat before it has a chance to harden. Whenever you’r working on a horizontal surface give it a check every 15 minutes, for the first hour or two, and add more gel coat as necessary. Follow the manufacturer’s instructions and look out for these common mistakes, and your gel coat repair should come out just fine. Fail to do so, and you might regret you started this project in the first place.


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  1. Roger D. Clifton

    How much and what kind of hardener do you use for gel coat? I ordered gel coat from my boat’s manufacturer, but it came with no hardener, information on use, or gel coat/hardener mixing instructions. Any information will be appreciated. -rdc-

    • LRudow

      Hi Roger – unfortunately, it will depend on the specific product and will vary from manufacturer to manufacturer. It’s a darn shame instructions weren’t included! Have you tried searching through their web site? That said, as a general rule of thumb, about one half ounce of hardener is required per quart. But before using this general guideline, I’d make a serious effort at seeing what the manufacturer recommends. Good luck!

  2. Roger

    Thanks for the information. I will contact Ranger. The problem is I’ll just be using very small amounts to touch up very small areas. (i.e. screw holes and a hole where cables were ran to depth finder, in addition to other small bumps and bruises. The largest area on the bottom of the hull is an area I fiberglassed measuring 4″ X 10″