All about polyester on board
In the 1960s, a revolution in boat building began. Prior to this, wood or steel was commonly used in the construction of boats. Things started to change in subsequent years, and plastic reinforced with fine glass fibres became more popular. Commonly known as fibreglass (glass-fibre reinforced plastic), or GRP for short, this new material made hulls lighter and shortened production times. For the first time GRP boats could be effectively produced in series, making them much cheaper. As a result, new customer groups emerged in what had until then been a very exclusive sector.
The polyester-based GRP used in these boats has long been one of the most popular materials and still dominates the boatbuilding industry today. Even though a successor has since been developed in which the glass fibres are impregnated with epoxy resin instead of polyester many boats on the second-hand market are still made with polyester resin.
In short, GRP can be made with both epoxy resin and polyester resin but since this guide is about polyester products, we will focus extensively on the topic of polyester GRP here. You can find information on epoxy-based GRP in our epoxy guide.
What is polyester GRP?
Polyester-based glass fibre reinforced plasticis exactly as it sounds: Polyester resins are used to bond glass fibres together.
The glass in question does not come in one piece, like a window pane, but instead is pulled into a very thin thread or fibre. It is this fibre that gives GRP both its tensile strength and high elasticity. It allows it to absorb large concentrated loads and distribute them over the entire surface. Depending on how the glass fibres are aligned, GRP has different properties. When working with polyester resins, one usually uses a random fibre mat that is produced in a spraying process. The glass fibres are chopped into small pieces and then held together by a binder. These fibres, which are usually a few centimetres long are dispersed randomly to form a mat.
This structure and the binder which is applied to hold the fibres together gives these chopped strand mats their strength. If you try to bend them before processing, they will break easily. However, during processing, the styrene contained in the polyester breaks down the binder. Fibreglass mats soaked in polyester resin are then soft and easy to shape.
The hull of a yacht is made of fibreglass mats and polyester resin in several layers to form a laminate. Laminating increases the tensile strength of the glass fibre mats while providing sufficient elasticity to withstand the occasional rough mooring manoeuvre. The biggest disadvantage of polyester-based GRP however, is that it is water permeable. To stop water getting through, the gelcoat provides a protective layer to the hull.
What is polyester gelcoat
To understand the gelcoat you have to turn the boat hull upside down: Boats are built in a negative or female mould from the outside in. n this mould, a porous layer of filler or putty serves mainly to prevent the polyester GRP from bonding to the mould and to make the hull easier to release from it. But that's not all: gelcoat is easier to sand to a smooth hull surface than the cured polyester GRP layer polyester GRP layer. This makes it easier for boat builders to produce a uniformly smooth surface on the hull. Polyester GRP must also be protected from water penetration, as polyester is not water resistant. To prevent moisture from penetrating the polyester GRP, a layer of topcoat is therefore applied during hull construction. This forms a waxy paraffin layer on the surface during curing, which is then water-repellent. – Damage to this final layer is a major cause of late osmosis damage, when moisture is gradually drawn into the hull over a long period of time.
In short: Gelcoat is a putty with the purpose of separating the GRP from the mould used in its construction and later serves to protect the GRP from moisture. .
What is polyester used for in boats today?
Over the past several decades, a vast second-hand market has emerged around durable polyester hulls. This market is full of boats that often have minor damage due to wear, scratches or knocks. Many amateur boat builders are on the lookout for such boats for their own yacht construction projects. Any costs for purchase and refitting are quickly offset by the selling price of a well-maintained boat. At the same time, you can learn a lot about boat building and craftsmanship.You can even earn a bit of money from your hobby, if you don't count the hours worked on the project.
Today, polyester is used mainly for small repairs to the surface of these boats and for large-scale repairs in the private sector. This is where it makes financial sense to use polyester as it is significantly cheaper than its successor, epoxy. However, whether this advantage can actually be sustained until the end of your own project depends on a few other factors in each individual case.
Working with polyester on board
The kind of repair depends on the type of damage. This is not often visible at first glance: Scratches in the gelcoat can go deeper than they first seem and damage to the hull also needs to be properly assessed and repaired accordingly. In the following section, we give some tips for everyday life on board and in the workshop on how to recognise damage correctly and repair it as necessary.
How to repair scratches in the gelcoat
Small scratches, caused by dolphins, fender rubbing or even a rough jetty, are usually only superficial damage on the outer layer of the hull. If only the (often coloured) surface of the boat has been damaged, but not the underlying, floury-white-grey gelcoat, this can be treated by polishing.A suitable polish is all that is needed to even out bumps and scratches that go down to a certain depth. However, if scratches go deeper into the gelcoat and have possibly already penetrated the top layer, new gelcoat must be applied. The best way to do this is to use a gelcoat repair or topcoat. This is because such, apparently, minor damage can cause moisture to penetrate into the underlying layers of the hull and begin its destructive work. This may show up years later in the form of a crumbling gelcoat around the what was previously a small spot, or in the worst case moisture will have moved all the way into the laminate and could cause serious osmosis damage.
Polyester gelcoat is available in all standard boat colours and an initial repair can easily be carried out at the mooring, at anchor and during the season, in just a few minutes. If necessary, the repair area can be lightly sanded down again later in winter storage and lightly polished with the rest of the hull. If the repair is done well, the damage will be hard to see, even to trained eyes.
How long can a gelcoat be used?
Gelcoat should never be used after it has hardened slightly. Otherwise, when applying a slightly crumbly mass, tiny gaps will appear through which moisture can penetrate and destroy the effect of the gelcoat. The resulting repair work is often much more extensive later on. Many manufacturers indicate minimum shelf-life for gelcoat. However, these dates are only valid if unopened containers are stored free of frost and heat.
Detecting laminate damage in the polyester hull of your boat
If the cause of damage to the hull is a hard knock or a deep scratch, the extent of this damage to the boat is sure to quickly go beyond the gelcoat. Because even if the damaged area only looks like a rough scratch it is possible that the hull itself has been weakened by breaks in the laminate. In the event of collisions, hard groundings or a really strong impact against a quay wall, the gelcoat around the damaged area must be removed and the GRP underneath also examined.
Use an eccentric sander on the gelcoat around the damaged area. Dust-free, Dust-free has a slightly yellowish, homogeneous colour. If the impact has been too much for the laminate to take, it will have been dented to such an extent that individual layers of the fibreglass mat will have separated from each other and the glass fibres will have lost contact with the polyester resin. These areas show up as noticeable white spots, which are also referred to in boat building as stress whitening in the laminate. The laminate appears to be intact, but is already much softer and its structure has been damaged. Delaminated fibreglass is also often found in the deck area of older ships, especially on the foredeck or the cabin roof, where work on the GRP was done sparingly in the shipyard when it was constructed.
How to remove stress whitening lines in the polyester laminate of your boat
Using very coarse (40 grit) sandpaper and an eccentric sander, carefully sand out all areas affected where the laminate is white. It is better to remove too much than too little GRP. For large repairs the removed polyester layers are nowadays often replaced with glass laminate soaked in epoxy resin. In the hull area in particular, this also provides osmosis prevention, as epoxy is water-resistant. Smaller repairs on deck carried out with polyester resin and fibreglass mats.
Take care of health and safety requirements: Glass fibre dust is harmful to your health! When sanding polyester and especially GRP, respiratory protection is particularly important. A cloth or dust mask won't be enough for this job! In the workshop, the area you are working on should also be covered with a tarpaulin, otherwise the fine dust will spread very quickly to the surrounding boats.
Working with polyester resin and fibreglass mats on board
- The area to be repaired must be dry and free of dust.
- Roughly cut fibreglass mats and place them on the repair area.
- Mix the required amount of polyester resin with the appropriate hardener.
- Wet the glass mat with the polyester resin using a brush and a little pressure.
- Depending on the hardener the curing process starts immediately after mixing, so work quickly.
- Sand off excess GRP after curing and match the repair area to the original hull shape with polyester gelcoat.
- Finally, finely sand and polish the gelcoat then wax or apply a layer of topcoat to achieve the best result.
What are the advantages of using polyester compared to epoxy for repairs on board?
- Polyester is less susceptible to processing errors than epoxy resins and cures even in unheated winter storage areas
- Polyester is cheaper than epoxy For large areas, the price difference can be worthwhile.
Disadvantages of polyester on board
- Polyester is not water resistant without a gelcoat or topcoat.
- Even after polyester has hardened, the solvent it contains, styrene, will still be emitted. However, the maximum permissible value and labelling requirements for styrene were significantly modified in 2017. Consequently, supposedly styrene-free polyester can be found, which, however, still contains small amounts of solvent. Unfortunately, this reduction also often reduces the quality of the GRP produced with it.
- The price advantage of polyester resin is often outweighed by the larger quantity required to achieve a strength comparable to epoxy.
- Polyester shrinks during curing and is therefore not suitable for filling gaps or precisely fitting components.
Working with polyester on your boat - summary
Anyone who buys a used boat or owns an older yacht will sooner or later have to deal with repairs to the polyester GRP and no doubt also to the polyester gelcoat. Small areas of damage can be easily repaired or even polished with gelcoat and topcoat. If you are skilled in handling polyester resin and fibreglass mats, you may be more likely to return to polyester for repairs in the future. Nowadays, however, epoxy has largely replaced polyester in boatbuilding. There are good reasons for this, and it is worth taking a look at the advantages of epoxy, the newer material, when it comes to major repairs.
Author Hinnerk Weiler
Hinnerk Weiler is a sailing journalist, long-distance sailor and real "old salt". An experienced sailor and expert in boat technology, when it comes to explaining the world of polyester on board, he knows what he's talking about.