Repairing Fiberglass On Boats (Complete Beginner’s Guide)

Fiberglass boats are arguably the best kind of boats you can own.  They’re lightweight, fun to use, and easy to maintain.

They’re even easy to repair, and in this post on repairing fiberglass boats, we’ll tell you exactly how to do just that.

Let’s get started!

Fiberglass Basics You Should Know

White (light gray) close-up fiberglass texture

Fiberglass is a wonderful material that can be used to repair all sorts of things.

Learn how to work with fiberglass, and you can repair boats, vehicles, bathtubs, and many other items.

You can even learn to create your own parts, canoes, and even coolers.  An entire lifetime could be spent learning how to build and repair things using fiberglass.

But first, you need to know how fiberglass works as well as what it is.  Fiberglass by itself is just a sheet of glass fibers woven or matted together.  By itself, it is not hard, waterproof, or useful for anything.

However, once you apply a chemical resin to the fiberglass, it becomes waterproof, hard, and incredibly durable.  It also remains lightweight, which makes it great for applications such as aircraft, automobiles, and boating.

The type of resin you use, the type of fiberglass you use, and the core material you use will determine just how hard and durable the constructed fiberglass becomes.

Types of Fiberglass Cloth

There are many different types of fiberglass cloths out on the market today.

Some of these cloths include chopped strand mat, biaxial mat cloth, woven roving cloth, continuous roving cloth, carbon fiber cloths, and the list goes on and on.

This being said, there are two types of fiberglass cloths that you’ll be working with:

  1. Fiberglass fabrics, or cloths
  2. Fiberglass mat.

The fiberglass fabrics will be your main structural material, but you may end up using a fiberglass mat to help you work around turns and to help waterproof low-stress areas like the live wells.

With these fabrics, you’ll also have different weights.

These weights will help determine how thick and strong the material is.  Popular boating fabric weights are 10 ounces, 6 ounces, 4 ounces, and 1.5-ounce cloths.  These fabrics are sold by the yard and generally come in widths between three and five feet.

The type of fiberglass fabric you choose will be determined by the application you’re using it for.

Also, lighter cloths can be applied in multiple layers, so if you need more reinforcement, you can always combine a 4-ounce cloth and a 6-ounce cloth in two different layers to provide the thickness and strength of a similar 10-ounce cloth.

Types of Resin

There are a lot of different resins out on the market these days, but the two you’ll most likely be working with are either epoxy resin or polyester resin.  Epoxy resin is more expensive than polyester resin, but it is often considered stronger.  It is also more versatile as the epoxy resin will bond well over polyester resin, but polyester resin may not work well over an epoxy.

This being said, different epoxy materials will work better with different core materials.

For example, the polyester resin will melt some foams and plastics.  I found this out the hard way when I mixed some epoxy resin up in a plastic cup.  The resin started leaking right through the bottom of the cup, and I was very thankful that I had decided to play with the fiberglass outside.

Some other types of resin you might run into are polyurethane resin and silicone resin.  These resins are even more expensive than epoxy resin, and it doesn’t really make sense to use them for boat repairs anyway.

Core Materials

The core material is just the material that is encapsulated by the fiberglass cloth and resin.  Core material on boats is usually balsam wood or a foam core.  However, almost any material can become a core material.

For example, I’ve applied several different types of fabric with polyester resin over the top of pieces of cardboard.  The fiberglass and resin are so strong that it turns the cardboard into a hard shell that you can use to make many different things, including small boats.

If you ever need to do a small one-off project that does not require structural support, think about using cardboard as a cheap and easy alternative to creating a mold or using traditional core material.

This being said, your best bet is to use a core material that is meant for your particular application.  For obvious reasons, being out on the water in a damaged balsa wood and fiberglass boat is a whole lot safer than being out there in a damaged cardboard boat.

Also, keep in mind that you don’t always need a core material.  Sometimes you can create a mold that simply holds the fiberglass and resin together until they dry and become components themselves.  Many fiberglass hulls are built using molds that consist of a thick layer of fiberglass and resin, and they are designed to be very strong with many layers of built-up woven fabrics and mats with no core material.

Camper shells are also built this way without any core material.  If you’re curious about what this looks like, just do a quick search for fiberglass campers on this website, and you’ll see plenty of examples.

When you’re doing repairs on an existing fiberglass boat, you may not be using any core material either.

This is especially true if you’re just repairing a crack or hole in a non-load-bearing area.  In this case, you’ll just be applying cloth over the crack and sealing it with resin.

How Is The Fiberglass Applied?

There are a few different ways that fiberglass can be applied.

You can:

  • Roll it on with a paint roller
  • Paint it on with a brush
  • Spray it on with a spray gun.

When doing fiberglass boat repairs, you’ll most likely be painting or rolling it on in small batches. 

The way you’ll do this is you’ll mix up a small amount of resin that you can successfully paint on over the fiberglass within about 15 to 20 minutes.  This is about how much time you’ll have before the resin begins to dry and harden.  If you mix too much resin, you’ll end up wasting it, so it’s important to start small until you get a feel for working with it.

Also, keep in mind that all of your fiberglass should be laid out over the area you’re repairing before you begin mixing your resin.

Once your resin becomes active, you won’t have time to go back and cut out pieces of fiberglass.

General Tips for Working with Fiberglass

female worker making fiberglass to fix a boat in workshop Marine shop worker making fiberglass to fix a boat in workshop

As we alluded to earlier, fiberglass must be mixed before it can become active.  This is usually done with a hardener.  The manufacturer will tell you just how much hardener needs to be added to the resin.

Gel coats can also be added to the resin, but this shouldn’t be done until you are on your final layer.

If you apply the gel coat too early, you’ll end up having to sand the layer down before you can add any additional layers.

Also, keep in mind that fiberglass resins are meant to be used at different ambient temperatures.  If the temperatures are too high or too cold, the resin will not cure properly.

You’ll know the resin didn’t cure correctly if the fiberglass feels tacky afterward.

Safety Tips for Working with Fiberglass

Fiberglass resins and hardeners can be toxic, and it’s important to keep them off of your body and out of your lungs.

Personally, I only work with fiberglass outside, but any well-ventilated area will do.

Also, if you’re working with polyester resin, you’ll want to wear a respirator.  Not only is polyester resin toxic to breathe, but it smells awful.  The smell is so bad that I won’t even leave the closed bottle of polyester resin inside my house for fear that it will make the house smell bad.

You don’t want to get resin on your skin or your eyes either.  Wear protective clothing, gloves, and safety glasses to prevent this from happening.

For additional protection, you can also get a lotion that will work to coat your hands.  This way, if the resin gets through your gloves, you’ll have an extra layer of protection between the resin and your skin.  If the resin does end up getting on your skin, be sure to wash it off immediately. 

People have been known to develop allergies to epoxy resin, and once you get this allergy, you’ll never be able to be around epoxy resin again.

Fiberglass Buying Tips

  1. Always buy fiberglass by the roll. Unfortunately, many people will try to sell you folded pieces of fiberglass because it is cheaper and easier to pack, ship, and store it this way.  Sadly, folding fiberglass cloth can weaken the strands, so when you buy fiberglass that is folded, you’re buying cloth that is already damaged.
  2. Buy the hardener with the resin. It’s always easier just to buy the hardener and gel coats with the resin.  Different resins will need different amounts of hardener, and the manufacturer will usually sell the two as a kit.
  3. Get good mixing cups. A poorly mixed resin will not work properly.  Get good mixing cups so that you can accurately mix the resin and get the most benefit from it.
  4. Buy it locally (at first). A local marine store owner can often offer you valuable tips for working with fiberglass, and they can help tell you where you went wrong when you run into trouble.  You’ll probably end up paying a bit more for your resin, but you’ll often end up with some good and free advice.
  5. Buy the right fiberglass for the job. When you go to buy fiberglass, you’ll notice right away that chop strand mat is dramatically cheaper than all of the other types of fiberglass.  There is a reason for this.  Fiberglass mat is not very strong, and it can be difficult to work with.  Only use fiberglass mat when the job specifically calls for it.  Resins are expensive, so there isn’t any point applying them over inferior fiberglass cloths.

Repairing The Hull

Before you decide to repair your own boat hull, you may want to do a reality check.

Is the boat hull severely damaged?  Is your boat an ocean vessel that is going to be put under a lot of stress?

If you answered yes to these questions, you might want to save this project until you have a lot more experience with fiberglass repair.  In this situation, a bad repair job could put your life in jeopardy, so the money you save will never be worth it.

This being said, there are a lot of small hull repairs that you could tackle yourself without much risk.  For example, a small hole in a small boat that you only take out on the lake shouldn’t be too much for you to handle.  Small cracks or holes in the areas above the water line might not be a big deal either, but please use your best judgment.

Repairing Holes in The Hull

To repair a hole in the hull, you may need to get some new core material.  Try to match the new core material with the existing core material.  Make sure the remaining core material is dry.

Once you have the core material, you’ll need to fit it into the hole.  Next, measure and cut your fiberglass cloth.  Lay the fiberglass over the area, using fiberglass tape if you have to.  Once everything is in place, mix up your resin and apply it over the damaged area.  You may need to do multiple layers.

For professional repairs of ocean-going hulls, the surrounding fiberglass is ground back on a slope of 1:12 to make a large area to be repaired; a 1/2″ thick fiberglass structure would be ground back 6″ in every direction from the hole.

The patch is then built-up with increasingly larger pieces of fiberglass mat and resin.  Again, you need experience before trying this.

Your final coat should be a gel coat that matches the color of your boat.

Alternatively, you could repaint the boat after the repairs have been made.

Repairing Cracks

Be careful when repairing cracks in your hull, as they could be a sign of bigger structural issues.

For example, a balsa boat with cracks in it could have rotten wood inside the hull.

However, if you just see some cracks from when you accidentally backed into a tree last week, feel free to try to take on the repairs yourself.  You probably won’t need any new core material, but you will need some fiberglass and some resin to patch up the crack.

Measure, cut, and layup your fiberglass before mixing your resin, and you shouldn’t have any trouble with the repair.

One thought to keep in mind with a repair like this is that you’ll probably spend more time sanding and painting than you will be doing the actual repairs.  The more time you spend sanding, the better the boat will look when you’re done.

Repairing The Floor and Deck

The first thing you should do before making any repairs to the deck or the floors is to check to see how severe the damage is.

Often-times, decks and floors are made from wood, and if they are spongy or have holes in them, the wood is probably rotten underneath.

Before making any repairs to the fiberglass, you’ll need to take out this rotten wood and replace it.  What seems like a small repair, in the beginning, can often end up being an extensive repair.  If you don’t believe me, just look at all the free “project boats” listed on Craigslist.

Oftentimes you’ll see that the person giving it away received the boat for free himself, thinking that he could do the repairs and get a free boat.

Once he started to tear the boat apart, he realized just how much work and money would be needed and decided to give up on it.

Once you do know what you’re in for, start replacing the decking and flooring, making sure to keep the materials dry until they are encapsulated with new fiberglass and resin.  Also, be sure to make a note of where the different pieces of hardware are connected to the deck, as you’ll need to put them back on when your repairs are complete.

If you’re careful, you can use the old decking that you pull off the boat as a template for creating your new decking.

Once your new flooring is complete, secure it to the boat and begin placing the fiberglass cloth over it.  Be sure to use a good amount of fiberglass as the decking will need to be strong enough to support people walking over it.  Mix up your resin and apply it in small batches.

If you have a large area to do, you’ll be tempted to make larger batches but don’t do this as you’ll only end up rushing the job or finding the resin sets up prematurely.

Flooring Alternatives

Another thought to keep in mind is that you don’t always need to replace your wood flooring with wood.  Some people will skip the fiberglass altogether and simply install an aluminum floor instead.

This can be useful in certain applications but not very useful in others, so you’ll need to analyze your particular situation before you decide to make any changes.

If you do decide to skip the fiberglass, just be careful to find out what changes this will have on the weight of your boat.  Making a heavier boat could reduce your boat’s max capacity and affect the way your boat sits in the water.

Repairing The Keel

The keel is an integral part of a boat.  On a sailboat, the keel keeps the boat upright in the water and helps to keep it stable in heavy winds.

Being at the bottom of the boat leaves it much more vulnerable to scraping the ground and running into rocks.  For this reason, a keel can often take damage even when the rest of the boat remains trouble-free.

The question is, do you want to repair this damage to the keel yourself?  

If you have a small recreational boat that you can easily bring up on land, I’d say go for it.  You can flip the boat over in your backyard and spend as much time as you need applying a new resin and gel coat to the keel.

For boats that you know will be scraping the bottom again, you may want to apply a keel guard as well.  This will help protect your keel, so you don’t have to constantly make repairs each season.

Repairing The Transom

A solid transom will ensure that your boat’s engine doesn’t fall into the water.  If you’re going to make this repair yourself, make sure you’re prepared to do it properly.

Oftentimes the core of the transom needs to be replaced, and you’ll want to make sure you replace it with a material that is as strong or even stronger than the original.  This is especially true if you’ve upgraded the original boat engine to something bigger and heavier.

Once you’ve replaced the core material, the steps for making the repair remain the same.

Apply your fiberglass boat cloth and use as many coats of resin as you need to get the job done.

This repair includes a lot of hard edges, so you may need to overlap your layers of fiberglass at the top to get a good seal.  Take your time and make sure the fiberglass wraps properly, or you’ll end up having to do a lot of sanding at the end of the job.  Also, be careful to account for where the engine will be mounted so that you can match the hardware appropriately.

Repairing Hatches

Hatches are easy repairs, and they’re great for getting started with fiberglass.

When replacing a hatch, use the existing hatch to make a template for the new one.

The new hatch should be easy to make, and you won’t have to use a lot of thick fiberglass cloth since it won’t be the main structure of the boat.

The main thing to keep in mind when doing this repair is that the hatch will need to be sealed properly afterward.  Oftentimes, people will do a great job repairing or replacing the fiberglass hatch, but they won’t seal it properly afterward, and they end with a leaky hatch.

Repairing Scratches

Scratches in the fiberglass usually don’t require extensive repairs, and the cost is usually minimal.

The reason for this is that the fiberglass does not need to be replaced, and the core material is not yet affected.

Unfortunately, some deep scratches in the gel coat can end up creating a lot of prep work.  You’ll end up needing to do a lot of sanding to get the boat to look new again.

You’ll also need to do a good job matching the color.

If your boat is new, this may not be a problem, but if your boat has been out in the sun for a while, you won’t be able to use the original boat color as it will end up being too dark.

You’ll have to color match the boat, or you’ll have to resign yourself to painting the entire boat over again.

DIY Costs vs. Professional Costs

Doing repairs yourself will usually save you money, and they can sometimes even save you on time.

However, some jobs are too big to be done by beginners, and some jobs just take too much time for the DIY repair to be worth it.

I’ll try to give you a breakdown of some of the costs here so you can decide for yourself whether or not the repair is worth paying for or doing yourself.

Example costs from

  • 6-ounce fiberglass cloth 50” wide and 5 yards long. – $25.00
  • 1 gallon of polyester resin with hardener.  – $39.00

With this much cloth and resin, you could do a lot of small repairs of cracks and holes.

A simple hole or crack repair might cost you $50.00 each, so you’d definitely save money on smaller repairs, and you may even have some extra fiberglass left over for the future.

For a balsa core deck repair, you might want ¾ inch core.  This costs about $50.00 for a 2’x4’ piece.  You’ll also need a 10-ounce fiberglass cloth and polyester resin.

This repair might end up costing you a couple of hundred dollars, but it could save you a thousand.

Foam core used for repairing heavy damage to the hull of a sea-going vessel will need to be very strong.  Not only this, but you’ll need to consult with a marine technician to determine what the best combination of foam, resin, and fiberglass should look like.

Between material costs and consulting costs, you’ll end up spending thousands of dollars.  You’ll also end up spending a lot of time, and you may devalue the worth of the boat since a buyer might not feel safe buying a boat that was repaired by an amateur.

In this case, you may be better off paying a professional boat repair company to do your repairs.

Final Thoughts

Working with fiberglass can be tricky, but it can also be a lot of fun.

Learn how to work with fiberglass, and you’ll find an almost endless number of projects to take on.

Just keep in mind that working with fiberglass is not cheap or easy to get into.  If you decide to take on fiberglass projects, be prepared to spend some time and money doing so.

Also, be prepared to mess up a few projects while you’re learning.

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