Filling a hole in your hull

April 17th, 2010 by admin Leave a reply »

So, you’ve removed a sea toilet and have a seacock sat there doing nothing, or perhaps you’ve moved some through hull transducers, and now you’re left with an enjoyment compromising hole in the bottom of your GRP boat. Fear not, it’s dead easy to fill such a hole, and hopefully this article will show you how.

There are a couple of methods available for this job, but in my opinion this method is the safest.

Items for the job

Polyester Resin (circa 100ml for a small hole)


Chopped Strand Mat

West System Epoxy

Filler substrate (west system do this too, 404 high density filler)

Gel coat repair kit

Drill with wire brush attachment

Angle grinder with a pad

A few other assorted bits (mentioned in this article)


So, you’re currently sat looking at the great hole in your boat. It’s going to look a lot worse before it gets better, but don’t worry. I know attacking your boat feels horrible, but this will give you solid repair, so have faith in your initial destructive efforts.

The first job is to taper the hole on the outside of your hull. The easiest way to do this is with an angle grinder. It’ll rip through your anti foul, gel coat, and GRP with ease.

Excuse the crude picture, but this is basically what you’re aiming to achieve.


Once you’ve ground out the taper, grab the drill with the wire brush attachment, and rough up the surface of the exposed GRP. It should look something like the following when you’re done. Don’t worry about scuffing up the surrounding gel coat.


The next job is to clean up the inside of the hull. If your boat is painted you need to remove about a 8inch square area around the  hole. The best, and indeed cleanest way to do this that I’ve found is to use the wire brush again. You can use a power sander to but it takes forever and kicks up a load of fine dust. Use the angle grinder at your own peril, because you will be cleaning up dust for days afterwards!

You need to take the area down to bare GRP, and again make sure it’s rough, which the wire brush will do in one stroke.

Once the area is cleaned up, use a paintbrush to ‘paint’ some acetone over it all. This will make sure the surface is prepared to make a good bond with the polyester resin.

Before we can resin, however, there’s another preparatory step. You’ll need a 6” square of cardboard (cereal box type of stuff), some Cling film, and a circle of wood roughly the diameter of your hole.  Don’t worry if it’s a bit smaller. The easiest way to make a suitable wood circle is with a hole saw and a scrap of plywood. Make sure the wood is thinner than your hull by a few millimetres.

Place the wooden circle on your cardboard, and wrap a single layer of Cling film over it. A friendly word with the staff in the marina café should source these items for you :)

The finished article will look something like this.


Using some Duck tape, stick this to the outside of the hull, with the wooden bung located in the hole. It’s simply to stop the chopped strand mat from being push too far through the hole during our next job.

So, back inside the boat, cut some squares of CSM (Chopped Strand Mat). Start with a bit that covers the hole with a couple of inches of edge, and cut each piece slightly bigger, until the final piece is the size of your prepared area. 5 or 6 layers in total will be enough. Next, mix up about 100ml of polyester resin, and using a paintbrush, paint the entire area with the resin.

Take your smallest piece of CSM and place it over the hole, then ‘stipple’ it with the paintbrush, adding more resin until the fibres are clear. White fibres are weak spots, so make sure it’s all clear and well saturated. Next add the slightly bigger piece and repeat.

There’s one more tool you need, and it’s one of these.


It’s a roller, and they’re available from all fibreglass suppliers. Don’t try this job without one, it’s important since it will get the air out of the layup and create a safe bond between the layers. Believe me here, you NEED one of these. Every time you add a new layer, give it all a good rolling over until it all looks wet with no air bubbles.

Once you’ve added your final layer of CSM, the repair should look something like this.


Note: Mine isn’t square because I was also repairing a smaller drill hole further up the hull.

Leave this to “go off”. The time this takes will depend on the temperature, but it’s a good idea to leave it overnight if you can.

We’re not far off now, only one major bit of chemical warfare left to inflict upon your boat, and that’s the West System epoxy. I use epoxy because, unlike polyester, it’s waterproof (that’s why your boat has gel coat, to protect the porous polyester). We’ll be gel coating to finish anyway, but epoxy provides a stronger bond, so that’s my gunk of choice.

On the outside of the hull, remove the Duck taped wooden bung, peel off any stuck Cling film (if there is any), and prepare the tapered surface with a final light scoring with the wire brush. Score up the outside of your new polyester resin too, then clean the area with acetone. If you didn’t let your polyester go off properly, then this is going to make a total mess of the job, so don’t be tempted to rush things.

Mix up some West System (again, about 100ml), and using a paint brush paint the entire area with epoxy resin. Now, add your filler substrate to the epoxy mix until it’s the consistency of double cream. Paint this in to the repair, making sure you get all the nooks and crannies.

Now add more filler until the epoxy mix is more like toothpaste, and smear a load into the hole using the paint brush. Take a piece of cardboard or plastic with a flat edge and smear it up the contour of the hull to smooth out the repair. It doesn’t need to be totally flush or ever so neat, the main thing to worry about is making sure it’s properly filled with no air gaps. If you have to do this in two sittings, then so be it. Don’t worry. If you’re a bit slow and the epoxy starts to gel, then stop what you’re doing, and mix up a new batch. It’s easy to make a mess of a job by trying to race against the chemical reaction, although the West System mix should give you plenty of time.

Once that’s done, the repair will now look something like this


Wait until it has gone off properly (not just gelled) and then sand it flat.

To finish the job, apply a layer of gel coat repair, then sand and polish as required.

1 comment

  1. Niall Kelly says:


    Thanks for that. Our boat will be coming out of the water this winter and there will be three skin fittings to be removed and I’m not exactly looking forward to that job, but you’ve made it sound terrifically easy!