In part 1 we walked through the design, part 2 covered the making of the plug and in part 3 we’ll be covering how to make the mould and hull by learning about:

  • Applying a gelcoat
  • Choosing your fiberglass
  • How to fiberglass
  • How to separate the fiberglass from the plug/mould
Creating a mould from a plug.

Preparing the plug

The first task was to wipe down the plug with acetone so that there was a clean surface ready to start fiberglassing. Once this was complete the most crucial step could be carried out, adding a separating agent! If this isn’t done correctly the fiberglass will bond to the plug, so in my case I used several (around 6 or more) coats of wax, each with several hours/days between coats and after each coat I buffed it.

Applying wax to the mould.

When I first started buffing the wax, it came off in clumps, which is common if you use too much. However what I discovered was that the material you use to buff the wax is equally important and apparently an old sock won’t cut it. In the wax tin was actually a sponge, which I just thought was the lid and when I buffed it with this there were no clumps.


To make the mould, we first applied a layer of gelcoat to the plug. It’s the gelcoat which produces the smooth shiny surface once you’ve finished fiberglassing. In our case because we’re fiberglassing on the plug, the smooth surface will end up on the inside which is what you want for a mould. When applying gelcoat you need to apply it liberally and make large brush strokes as this helps to produce a smooth coat. Be careful to minimise any drips because if you do need to tidy up the gelcoat it needs to be done quickly whilst it hasn’t started to cure. If you try to make changes past this point then you’ll actually put a mark on the gelcoat. To start the first layer of fiberglass, we waited till the gelcoat was tack dry which can be tested by seeing if you can stick a fingernail mark in it.

A photo of the gelcoat applied to the plug.
Gelcoat once it’s been applied to the plug.

Just a note on temperatures, whenever dealing with gelcoat and fiberglassing it’s important to make sure you know the temperature your fiberglassing in. This will affect the amount of catalyst you need to add and how quickly it will catalyse. As my garage was at 0-5 degrees with no heating I managed to heat it up to 10-15 degrees and used an infrared thermometer to check the surface temperature.


When gelcoating, fiberglassing and using the associated chemicals you should be wearing full PPE – that is eye goggles, respirator and gloves. I’d recommend the 3M 6200M facemask with the 3M 6055-A2 Gas And Vapour Cartridges and 3M 5935-P3R Particulate Pre-filter connected with a 3M 5000 Series Filter Retainer. You need both the filter and gas cartridge so that it filters any fiberglass dust and also vapours from the chemicals. The polyester resin smells really bad and you’ll definitely want to be doing this outside in a well ventilated area and still with a respirator. 

Don’t underestimate the smell of polyester resin and wear correct PPE!

Even with doing the fiberglassing in the garage, my clothes had to be put aside as they just stunk. Fiberglassing is also really messy and the fibres get everywhere, which if mixed with resin can be like sharp glass splinters. Fiberglass particles are also an irritant so you’ll want to stay covered up or wear a decorating suit if you can.


Fiberglassing for the boat and mould both require the same set of techniques, except we’ll be using different layers of fiberglass. For the mould we used alternate layers of 300gsm chopped strand mat (CSM) and 600gsm woven roving. For the boat we used 4 layers, 300 gsm CSM, 600gsm woven roving, and then a 300/600 combination matt which is essentially the previous two combined. The idea behind this is that because the matt is already bound together it requires less resin and is slightly stronger.

Woven roving fibreglass up close.
Woven roving up close

We used a combination of CSM and woven roving as this is what traditional boat building uses. There are other materials that you can use such as biaxial matt but you need to choose the material suitable for you application as the different materials have different strength properties. For example CSM is fairly weak and the direction of strength is omnidirectional whereas biaxial matt is stronger in certain directions. So if you decide to lay something like biaxial matt consider alternating the direction of the layers so that the strength isn’t always in one direction. 

Calculating Weight

You’ll need to calculate how much resin is required and this will also help determine the estimated weight of your fiberglass product. You’ll need to add up the weight of fiberglass for your area, and then add resin at the correct ratio depending on the type of fiberglass. Typically chopped strand matt requires 2x resin and woven roving requires 1.2x resin.

Total Fiberglass weight = Fiberglass weight (gsm) x area
Total Resin weight = Fiberglass weight (gsm) x area x resin ratio
For example, the resin for a 300gsm chopped strand mat over a 5m² area would be:

300gsm x 5m2 x 2 = 3000g

Plan the fiberglass layout beforehand.

I found it useful to cut the fiberglass pieces to size and plan their placement beforehand. This is so you can lay as much as possible until the resin starts to cure to the extent you can’t use it. To cut chopped strand matt you can tear it really easily but for woven roving you’ll need to use some heavy duty scissors. If your scissors don’t cut well then you’re going to end up pulling threads from the woven roving and make a mess.

Testing the layout of fibreglass
Planning and testing the fibreglass layout

Resin and Catalyst

When you start fiberglassing it will take a few rounds to work out how much resin you need and can use before it starts to become unusable. I suggest starting with a small batch and bringing along a friend so one can mix whilst the other lays the fiberglass. Polyester resin is typically mixed with a catalyst (MEKP), and the amount of catalyst depends on the temperature, and how quickly you want it to cure.

An assistant mixing the catalyst and resin.
It’s useful to have an assistant who can mix the catalyst and resin whilst you fibreglass.

I’d recommend to measure the resin in one container and then mix in another. Otherwise you’ll go through measuring containers very quickly. If you actually leave a  small chunk of resin in the bottom that’s unused, wait till it goes hard, then just tap the container upside down and the hard block of resin will fall out giving you a clean container. This works best with plastic so you can flex it which breaks the seal between the container and resin. Recycled plastic soup containers are good for this! 

Re-using resin containers.
Once the resin sets in a container we can tap it out and re-use the container.

Also make sure you’ve mixed your resin thoroughly including the sides and bottom as you don’t want parts of the resin to not catalyse. As I was mixing small amounts of resin the amount of catalyst required was only a few mil. To measure this accurately I used syringes and I put tape around the markings as with repeated use the numbers would wear off.

Pro-tip, if measuring the catalyst in a syringe mark the levels with tape in case the markings rub off.

Laying the fiberglass

To lay down chopped strand matt you’ll need to apply the resin with a dabbing action. If you try and brush it you’ll pull the fibres apart as it’s held together temporarily with a powder that dissolves in resin. To ensure that the resin has penetrated fully and that there are no air bubbles you’ll want to go over it with a serrated roller. CSM is great for unusual shapes because with dabs of the brush or roller you can get it to conform to a lot of surfacesHowever you won’t be able to fiberglass around very sharp 90 degree corners as the fiberglass will pull away. Be careful not to get carried away and use too much resin as this will decrease the strength and make it brittle. Whilst you’re applying the resin you’ll notice it starts to change colour and when it goes gelatinous it will have reached the end of its usability.

Fiberglassing the boat mould.
The green is where the resin is fresh and as it starts to cure it turns yellow.

The next layer of woven roving can be laid as soon as the other is finished, but you shouldn’t apply more than 3/4 layers at a time. Fiberglassing creates an exothermic reaction and therefore if you apply 4 layers or more a lot of heat is going to be generated, which isn’t good.

When laying the woven roving you have to use a different technique to apply the resin. This time you can brush it on, but due to the nature of woven roving it’s harder for the resin to penetrate so you’ll have to work harder with the brush. Once that’s done you can scrape and apply pressure to the fiberglass with a plastic card which helps force the resin through but also allows you to scrape off the excess.

When comparing CSM to woven roving you’ll notice that CSM moves around a lot and pulls apart but the woven roving stays bound, but it doesn’t always contour well around certain surfaces. This is partly why I recommend cutting the pieces and testing beforehand, especially if you have complex surfaces, so you know how they fit properly.

Testing the layout of the fibreglass on  the back of the boat.
Cutting and testing the fibreglass on the back of the boat.

Whilst fiberglassing it’s important to make sure that there are no bumps. I was surprised of the affect a few stray fibres or a lump would cause. When the layers cure and you try to apply the next layer, the fiberglass wont adhere to the surface of the lump so it creates an air pocket around it, which will affect your structural integrity.

Watching out for bumps in the fibreglass.
Lumps in the fibreglass will need sanding afterwards, but to prevent this, make sure your surface is flat first.

Therefore once you’ve done as many layers as you’re doing in one session, and they’ve set, it’s a good idea to check for bumps and sand them down, this will ensure that your next layer bonds well.

Tidying Up

If you’re fiberglassing over an edge, like the flange of the boat, you’ll want to fiberglass a few cm’s over it and cut the excess material away afterwards. This is so you can ensure there’s a full layer of resin until the end. This is particularly important because I noticed on the corners that some parts are missing a layer or two of resin, making them vulnerable. So keep an eye on your edges!

I can guarantee that as this point you’ll be very messy especially with fibres from the chopped strand matt as they get everywhere! If you wait till they go hard you can peal them off your gloves. Personally I buy really cheap brushes and I’ve attempted to clean them but I find I can never get them to go back to a useable state so I end up binning them. Don’t forget to clean your rollers to, otherwise these will end up stuck!

Sawing the edge of fibreglass.
Clamped a wooden batten to the the edge and used it as a guide to cut a straight edge.

After repeating this several times the fiberglassing was finished and my mould was almost complete. We left the mould for several days to cure as although it can go hard very quickly the chemical reaction is still taking place slowly. I trimmed the excess material away using a saw and jigsaw, which in turn created the edge of the flange. I also lightly sanded the edges to make it safer to handle. I left a few rectangles protruding from the edge as this would make handles which would become useful when removing the mould from the plug

Removing the mould

Now for the most exciting part, trying to remove the mould from the plug! This is the time where you’ll find out if you did your waxing properly at the beginning. In our case the mould was difficult to separate and we tried numerous methods to release it.

We inserted wedges between the mould and plug but this didn’t initially work. The problem was further exacerbated when I hammered a wedge in and rather than forcing and popping the mould off, it actually lifted the plug off the base board that we initially screwed it to. We therefore ended up removing it ourselves, and tried all sorts of methods to get the mould released. We suspended it in the air hoping for gravity to work it’s magic…nope. We flipped it upside down and put pressure on the flanges, kind of trying to flex it. That didn’t work. What eventually worked was we just kept reinserting the small wedges and trying to break the seal. Eventually we heard the most satisfying crack and with a few more taps of a wedge it released.

Suspending the mould in the air trying to remove the plug.
Letting gravity do the work to remove the plug didn’t go to plan.

I was lucky it did release because the next plans were either to destroy the plug and literally remove it from the mould or drill and attempt to pop it off with compressed air. Which considering we’re designing a hull, a hole is far from ideal!

To make the actual boat hull I followed the same process except the actual fiberglassing was done on the inside of the mould, which was a lot harder to reach. The combination matt used for the final layer was a lot harder to get the resin to penetrate fully and so we actually applied some resin before putting down the fiberglass.

The final boat finished.

So there you have it, the mould and hull complete! For a time-lapse of the whole process check out the video below.

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