How to Repair a Fiberglass Boat Hull From the Outside
  • Post author:
  • Post last modified:May 6, 2021

Disclaimer: As an affiliate I may earn a commission on any qualifying purchases, including those from, at no extra cost to you – read more.

You’re out for a day of cruising in your new boat, weaving in and out of islands on your favorite lake. After a long day out on the water, you turn around to head back to the boat launch, only to hear the dreaded scraping and crunching of your boat against a shallow rock. Your heart sinks as you think of all the time and money it will take to repair your boat if you send it into the shop.

But what if you could repair your fiberglass boat hull all on your own? Sound crazy? It shouldn’t. While fiberglass boat repairs are a bit more complicated than those you’d find on wooden boats, most small dings, scratches, and even small holes are, indeed, repairable. To help you get started, we’ve got the ultimate guide to repairing a fiberglass boat hull from the outside, complete with tips and tricks to help get your boat back on the water in no time flat.

How To Find a Leak in a Fiberglass Boat

While some holes in boat hulls are caused by a catastrophic collision, many more seemingly come from nowhere. Often, these small leaks build up over time so slowly that you don’t know it’s happening until it actually does. This kind of progressive hole development can be scary, especially since water can start coming into your boat without giving indicators as to how or why got in there.

Identifying a Small Leak

boat leak engine compartment

If you’ve got a leaky boat, you first need to figure out where the leak is coming from, which can be trickier than it seems. That being said, there are some telltale signs of a leak that often happen before water starts flooding into your boat.

Often, with leaks, prevention or early intervention is key to reducing potential damage. Every once in a while, you’ll want to go around and inspect your boat, just to check to see how things are doing and if there are any minor problems that you can quickly fix.

When it comes to leaks, look for paint bubbling or breaking down near an overhead or deck on the boat’s interior. You might also notice discoloration or corrosion on some of your metal fixings. If some of your fixings have this issue and not others, you may have a leak on your hands.

Finding the Source of a Leak

Once you identify that your boat is leaking, it’s now time to identify the source of the leak. There are a few different methods that you can use, so let’s look at a few of them here for holes above the waterline:

  • If you have a buddy, you can ask them to stand inside your boat as you get a hose with a spray attachment. Once you have your hose ready, begin spraying water onto your boat, starting away from the suspected hole site and moving slowly toward it. Your friend on the inside of the boat can alert you when they start seeing water. Voila! You’ve got your leak.
  • Should water not be an appropriate method for identifying a leak in your boat, you can try to use a can of compressed air. Start by sprinkling some baby powder onto the outside of your boat and then use the compressed air to blow on the location from the inside. If you’ve found the source of the leak, you’ll probably find a disturbance in the baby powder on the outside of the boat.
  • If you want to get fancy, you can also get yourself an ultrasonic leak detector to find your leak. This technology basically transmits soundwaves through your boat and uses that information to identify where there might be a hole in your boat. That’s some pretty cool stuff.

All of those methods are fine and dandy for leaks above the waterline, but what do you do if the hull of your boat is leaking? Here are some suggestions:

  1. First and foremost, unless the source of the leak is obvious (e.g., a large hole), you’ll want to start by completely drying out the bilge. You can do this by letting your bilge pump do its job and then going in after it’s finished with a sponge and some towels to dry the place out properly.
  2. In your fiberglass boat, you’ll want to then check all of the various hull penetrations for potential leaks. A good flashlight or headlamp is indispensable here.
  3. If the leak source still isn’t obvious, you’ll want to start by checking out all of the various fittings in the bilge. Wipe the hull fitting off with a paper towel to dry it completely, shine a light on it, and rub your hands over the fitting. Often, your fingers will feel water coming in through a hole that you can barely see.
  4. Should the leak still not be apparent, you may have to look inside a closed compartment or under the edge of a hull liner. Sometimes, you need to cut a small access hole to check out these spaces and get a pump in there.
  5. If your boat is a powerboat with a double bottom hull that’s filled with flotation foam or liners, it’ll be pretty difficult to look for a leak without getting fairly destructive. So, if you’re still struggling to find the leak, you might want to haul the boat out onto land when the bilge is still full of water and investigate from the outside. You might find a trickle of water leaking out that’ll give you an idea of where the leak is coming from.

How to Repair a Fiberglass Boat Hull

Fiberglass Hull Repair Work

Once you find your leak, it’s time to fix your boat and get it back in great working order. While many people take their boats to the shop for a professional repair, it’s often not necessary to do so if you have a few simple skills. Additionally, even if you’d prefer to leave it to the pros, all boat owners should know how to do some simple hull repairs, just in case you need to do one in an emergency.

To help you get started with your repairs, we’ll walk you through some of the basics of fiberglass boat hull maintenance. We’ll talk about different products you can use to repair your boat, how to take on some emergency repairs, and the difference between in-water and out-of-water fixes. Here we go!

Amazon has TotalBoat Thickset Clear Expoxy – Check Pricing

Emergency vs Long-Term Repairs

If you’re out and about on the water and you find a significant leak, chances are pretty good that you’ll want to fix that leak, pronto. Sizeable leaks can cause some serious damage to your boat or even put you in danger, so anyone operating a boat should know how to do an emergency repair to their hull, but they should also know how to recognize a repair that can wait until you’re back int the marina. Let’s take a look at the difference between emergency and long-term repairs now.

Emergency Repairs

Emergency fiberglass boat hull repairs

Emergency repairs are the kinds of fixes that need to happen at that very moment or you could be at risk of having a nice day on the water turn into a disaster. During an emergency repair, your job is to slow the leak down as much as possible so you can hobble back to a safe haven, be it your home marina or another port.

A good option during an emergency is to use a “collision mat,” which is basically a heavy-duty tarp with lines attached to it that can be pulled under the boat. When you pull the collision mat under your boat and cover the site of the damage, you can slow or perhaps even stop the leak to allow your bilge pump to catch up. Of course, using this method requires that you have a collision mat on board, though you can find these at most major marine stores.

Does Flex Seal Work on Fiberglass Boats?

If you’ve watched a bit of TV at some point during the last five years, you’ve probably heard of Flex Seal, the self-proclaimed super tape that can be used to fix any surface. At one point, one of the Flex Seal commercials showed the company’s spokesperson fixing a boat with the tape alone, encouraging millions to think that they, too, could save a lot of money and time by using Flex Tape on their boats.

While Flex Seal can, indeed, be used to fix a boat, it’s really more of a band-aid than a long-term fix. If you really care for your boat, you’ll want to take the time to do a thorough proper fiberglass repair to make sure it can withstand the test of time. But, since Flex Seal does make for a great emergency fix, it’s worth carrying around in your toolbox, just in case.

Long-term Repairs

Unlike emergency repairs, long-term repairs are done with the goal of permanently fixing the source of the leak in your boat. These repairs are almost always more labor-intensive than an emergency repair as they’re designed to stop the leak, once and for all. Let’s take a look at options for long-term hull repairs.

In-Water vs Out-of-Water Repairs

If you need to repair a hole or leak in your fiberglass boat hull, you have two choices for how to proceed: you can take your boat out of the water or leave it in. Leaving your boat in the water has the clear benefit of saving you both time and hassle, but does make it more difficult to repair certain kinds of leaks. In fact, there are times where you’ll definitely need to remove your boat from the water. Here are some general guidelines for how to repair your boat in either situation:

In-Water Repairs

In-water repair jobs are usually reserved for when a boat leak is coming from a place above the waterline, where some quality caulk made for marine purposes can do the trick. Often this gets applied to cracks around window frames and other similar locations. Be sure to read the instructions of your particular brand of caulk so you give the material enough time to cure and stop the leak for good.

Out-of-Water Repairs

Repairing a Boston Whaler hull out of the water

If a leak is coming from below the waterline, you’ll probably have to haul your boat out to do a long-term repair. Even if a leak is easy to pinpoint (i.e. coming through a fitting), you’ll almost always have to replace the entire fitting to permanently stop the leak. After replacing the fitting, you can add a coat of sealant for extra security.

If the leak in your boat is coming from a crack or hole in the fiberglass of your hull, you’ll need to either send it in for repairs with a professional or take the time to make the repairs yourself. Should you choose to repair your boat on your own, you’ll need all of the following materials:

  • Rotary tool
  • Sanding block
  • Safety glasses
  • Buffing compound
  • Gel coat (see recommendation)
  • Acetone
  • Disposable spray gun
  • Gel coat reducer
  • Powdered fiberglass filler
  • Sandpaper
  • Wax

Once you have all of your materials, it’s time to get started with the repair. Here are the general steps you’ll need to follow to fix most gouges and small holes in your fiberglass boat hull:

  1. Clean your boat. The best way to start a repair is to give your boat a nice wash and make sure it’s super clean before you start working on it.
  2. If you’ve got a deep gouge or cut in your fiberglass hull, you’ll want to round out the edges and create a u-shaped groove by using a rotary tool. This will help eliminate sharp edges and make your repair look and perform better once all is said and done. Lighter scratches can be sanded out by using 80-grit, 150-grit, and finally 240-grit sandpaper.
  3. Mix up your powdered fiberglass filler with your gel coat to make a thick paste and stir. Once you’ve got the right consistency, you can apply it to the gouged areas. You’ll want to overfill your gouges slightly so you can sand them down after it cures.
  4. Once the fiberglass filler cures, which can take one to two hours, you can sand it down with 80-grit sandpaper.
  5. Next, you’ll want to mix up a new batch of gel coat powder (don’t add the fiberglass filler this time) and some gel coat reducer (follow the label directions) and load it into your disposable spray gun. Spray the newly patched area in small amounts.
  6. Once the gel coat has cured, you can sand it down with 600-grit and 800-grit sandpaper. If you’re really looking for a perfect repair, you can try using a 1200-grit wet/dry sandpaper for extra smoothness.
  7. Finally, get your buffer out and start bugging the new gel coat. You should start slowly, buffing small sections at a time with medium pressure that eases up as you buff.

Congrats, your fiberglass boat hull repair is done and it’s time to get back on the water! While fiberglass boat repair may sound scary and expensive, small and moderate gouges in your hull don’t have to be a nightmare. With the right tools and skills, you can find a leak and fix it before it becomes a problem. Happy boating!

John Allen

With more than a decade of experience cruising the lakes in my Crestliner Grand Cayman pontoon boat and my Boston Whaler, I now want to share everything I've learned with my community here at Boating Hub.

Leave a Reply