How to Patch a Large Hole in a Fiberglass Boat from the Outside

Although I am building a 16′ wooden sailboat, an ad for a damaged 20′ fiberglass boat caught my eye. When I drove to look at the boat I found that it did have severe damage on the hull bottom on both sides. The owner told me that he had run over a submerged tree. There was a hole on the starboard side that could have passed a soccer ball and another elongated hole on the port side that could have passed a basketball. He readily admitted that the reason he was selling the boat for a low price was that he had been quoted a repair estimate of $2,000 which would have been close to the value of this 25 year old boat. Essentially, the boat appeared to be totaled. However, in spite of the damage and the repair estimate, I purchased the boat anyway for $900 which included the trailer, a small outboard, a jib, a genoa, the original main sail, and a brand new $700 main sail. So, why did I buy it?

I was not only confident that I could repair the damage but I was also confident that I could do it for a lot less than $2,000. I figured I could do the repair for about $70 not counting paint or gel coat. The reason for the large difference is the method. A typical boat repair would use the direct method. This is where the inside edge of the hole is tapered and a piece of backing material is placed on the outside of the hull. This backing material would be something like cardboard. It would have to be able to bend but stiff enough to hold its shape across the hole. Then the layers of fiberglass are simply laid up on top of this backing piece. Once the layup cures, the backing material is removed and there is only a minor amount of filling and sanding necessary before painting. This is the preferred repair method. However, in the case of my boat, the holes were unfortunately under the deck liner. This fiberglass liner is reinforced with plywood underneath. A direct repair would have required cutting large holes in both sides where the deck liner is the floor of the quarter berths. And, this would have meant repairing large holes in the berth floors as well. So, the $2,000 estimate was not a surprise.

However, there is another method that is used for small holes when you can’t do it from the inside: the blind repair method. This method is basically the same as that used for patching a hole in plaster or drywall. This requires putting the backing piece on the inside instead of on the outside. However, since you can’t reach the inside, the backing piece has to be secured on the outside in some fashion. Typically, a piece of cardboard is put through the hole and then held in place with a string tied to a stick that spans across the front of the hole. The cardboard simply provides a backing piece to keep the wet plaster from falling out the back. Once the initial fill is dry the string is snipped off and the finish coat is applied. Repairing a small hole in fiberglass by the blind method is very similar. The only difference is that you first put a layer of glass fabric on the facing side of the cardboard so that it will stick to the inside surface of the fiberglass. Because the string is now in the way it is difficult apply additional layers all at once. It is easiest to allow the first layer to cure and then use it as a backing piece for the subsequent layers of glass fabric. For a small hole, this method works just as well for fiberglass as it does for plaster. You can see a good description of the blind method at the link I provided. Cliff Jacobson gives good detail and a nice illustration in section 2. Single-sided patch. This method is backwards of using an outside backing piece and the edge of the hole is tapered on the outside inside of on the inside.

However, this comparison with drywall falls apart when the hole is larger. To repair a large hole in drywall you cut out a patch that closely fills the hole. Then you glue a couple of backing strips of wood across the back of the opening to keep the patch from falling inside the wall. Then you simply screw the patch to the wooden strip or strips to hold it in place while you fill in the gap around the outside with heavy drywall compound. This works great for drywall because the wall is flat. However, this simply is not possible with the compound curves on the hull of boat. You can try scaling up the regular blind method but you risk having the cardboard buckle or bow out too far when you pull the string tight to make it conform to the proper shape. Adding additional strings also creates other problems.

Naturally, I first had to cut away all of the cracked and broken edges of the fiberglass and sand the outside tapering in. Then, I did a lot of thinking and some trial and error and finally came up with a method that worked. I had read Cliff Jacobson’s tutorial which is a great starting point, but it was clear that his method would not scale to a large hole like my boat had. Cardboard was too flimsy while luan was too stiff. Foam cored posterboard about 3/16″ thick seemed to have the right stiffness but was still flexible enough to bend and conform to the hull shape. And, you can get this at most places that carry posterboard. I used a large sheet of paper to trace around the edge of the hole. Then I cut out this shape and traced around it onto the foamboard. I then allowed an inch on every side and cut out the larger shape. Then I checked to see if it would actually fit into the hole. I was able to put the piece in sideways and then rotate it to the correct orientation because the holes were oval on both sides.. However, if your hole is not oval shaped then you will need to do this in two steps. You will have to cut the foamboard in two and do each part one at a time. The problem is that if you cut the piece in half it will bow out too far in the middle. So, you either have to cut closer to one edge or you have to put a reinforcing strip of wood on the back of the edge to keep it from bowing so far. This may take some trial and error though to get a piece that will flex but not flex too far. I then used the foamboard as a template to cut out the first layer of glass cloth. You can cut the glass cloth about 1/4″ less at the edges to make handling easier. However, if you are using two pieces you want the mating edge to be longer instead of shorter so it can wrap around to the back.

Using popsicle sticks to prevent the string from pulling through works well. However, one string is not enough for a large hole. You need strings spaced in rows about 4″ apart to be able to properly tension the foamboard to make it curve the right way. This is a problem for the first layer because with a small hole you simply wet the glass fabric with resin and stick it to the cardboard before you put the string through. Unfortunately, with many strings there isn’t enough time to do this after wetting the glass fabric. So, I came up with the idea of using heavy thread and a darning needle and stitching the glass fabric to the foamboard. I haven’t tried it, but you might also be able to use a spray adhesive to stick the glass cloth to the board. You just need to check and make sure that the adhesive is strong enough. After you have the glass cloth secured you just tape popsicle sticks to the back and then use a large needle to loop a string around each one. You’ll want to draw lines for the sticks so that the strings fall into rows which will match with what you tie them to on the outside. You can get these craft sticks at a lot of hobby stores if you haven’t saved enough popsicle sticks. I broke mine in half since a whole one wasn’t large enough for two strings but was bigger than I needed for one string. If you are doing the patch as two halves then you want the glass cloth to be longer where the two edges meet. Wrap the edge around and tape it or glue it down on the back so that you have a clean edge for the second piece to butt up against.

I had already tapered the outside edge of the hole but I also sanded the around the inside to make sure that I got good contact. I got large 1/2″ thick dowels to span across the hole and blocks of wood to hold them up away from the surface. Without blocks of wood to hold them up, the dowels would hit in the middle because of the curve of the hull. It is difficult to be neat with when wetting the glass fabric. It is almost impossible to not get the string wet as the same time. I wet the glass fabric through and then spread some around the inside of the opening where the fabric would touch. I put the foamboard through and then started typing up the strings. You may only want to tie them once at first because it might take a couple of tries to get them adjusted right. When you get them the way you want then you can tie them twice and you are done until it cures. Obviously with two halves you would just repeat this. The string will also get resin in it so you may have to cut these off the dowel that spans the hole. Also you will have to snip off whatever string sticks up and carefully sand off any bubbles or places that stick up. Once you’ve gotten this far, you have a backing piece to complete the additional layers. Since the port side hole on my boat was long and tapered I had to do this in two steps and butt the edges together.

Of course, you have to decide what type of resin to use. You probably have a strong incentive to test and see if polyester resin will stick properly since polyester is about half the cost of epoxy. Simply put, epoxy will stick to either epoxy or polyester but polyester will only stick to polyester. This is because epoxy is a glue whereas polyester will only stick if the solvent can soften the existing polyester surface. Most boats are made with polyester resin so this will usually work however if you have any doubt you can always use epoxy. I did use epoxy with mine since I was already using epoxy for my wooden boat.

However, when the first layer cured I did have some gaps around the edge where it didn’t quite touch the inside surface. These needed to be filled before going any further. You can thicken some resin with fumed silicon (which is like dry snow) and fill in any small gaps before adding additional layers. If you have a gap bigger than 1/4″ then you should just take a small piece of glass fabric and wet it and then stuff it into the gap. This will be stronger than trying to fill a big gap with thickened resin since fumed silicon isn’t structural. Once any gaps are filled and cured you can then lay up additional layers. However, the next problem I encountered was working upside down. If you are patching a canoe you can just turn it over to put the work surface on top but I didn’t have that option with a boat weighing over 1,000 lbs. It can be done but you will find that you have the same problem as wallpapering a ceiling. If any edge hangs down it will simply begin to peel and keep peeling until if pulls off the whole piece. Each edge has to be wet and stuck to the surface and the fabric smoothed of any wrinkles. It is picky work and requires patience and persistence but it can be done. Since this is messy I wore protective rubber gloves similar to dishwashing gloves but not made of natural rubber. You can find these in the finishing section of hardware stores and these gloves will say that they work with solvents. I frequently had the wetted fabric laying on the back of my hand while I smoothed it into place with a plastic squeegee and the glove was the only thing keeping the resin off my skin.

I wanted to reduce the time to build up layers so I used thicker mat for the first layer. Most mat only works with polyester but mine was suitable for either polyester or epoxy. After this was in place I still had to cover it with another layer of fabric since mat will tend to fray and leave strands sticking up that will be in the way. Covering it with glass fabric left a smooth surface. Then I was able to stop and let this cure. You just continue this process until you build up enough to match the original surface. Sand out and fill any bubbles and fill any any low spots. You may also have to do a little sanding if you have any rough areas. In particular the cut edges of the glass fabric can leave stiff threads sticking out that interefer with additional layers. It is definitely more labor intensive than using a backing piece on the outside and putting in all the layers at once but when you can avoid cutting through a liner or other surface it can be the best option.

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