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Introduction
GRP Basics
(This page)
BACK TO FULL LIST OF PROJECTS
Introduction
GRP Basics
(This page)
This page explains what Glass Reinforced Plastic (Fibreglass) is, and how it is created and moulded to make a boat hull. There is also a section on the difference between ordinary Gelcoat & the variety called Flowcoat (Topcoat) as it is important to use them correctly when restoring damaged gelcoat.
Glass Reinforced Plastic (GRP) is also known as Fibreglass (or Fiberglass). It was originally developed in the UK during the Second World War (1940's) for radar domes on aircraft (GRP does not block microwave radar signals). After the war its first main civilian application was for boat building, where it gained acceptance in the 1950s, and now plays a dominant role. GRP consists of three distinct components:
The lay-up resin polymer alone is brittle and has a low strength but when it contains embedded fibres of glass it becomes strong, tough, resilient and flexible. A resilient material is one that returns to its original shape after bending, twisting, stretching or compression. These features are combined with lightness which make GRP ideal for making boat hulls, car bodies, baths etc.
The following section gives an outline of the stages in making the GRP hull of a boat. This also indicates the materials involved in maintenance & repair which are described in more detail on the other pages.
STEP 1 - Preparing the Mould
The mould has a highly polished surface and is coated
with Release Agent to prevent the GRP sticking to it.
The first stage is to prepare the mould. Often the mould itself is made out of GRP and taken from a wooden prototype of the boat. This was how the first moulds for the Lugger were made. A diagrammatic cross section of a hull mould is shown above. The surface of the mould has to be very smooth and highly polished. It is coated with a Release Agent (usually Polyvinyl Alcohol [PVA Release Agent]), to ensure that the finished hull separates easily without sticking to the mould.
STEP 2 - Adding the Gelcoat
The gelcoat has pigment added to give it colour.
Catalyst is added and it is then painted or sprayed on
to the surface of the mould. The catalyst initiates a
chemical reaction which gradually turns the liquid
gelcoat into a hard but flexible plastic. Curing is
'air inhibited' so that the gelcoat next to the mould
sets hard whilst the inner surface exposed to the air
remains tacky (only partially cured), until the lay-up
resin is added (See STEP 3)
Gelcoat Preparation
The first part of the boat to be built is the gelcoat
which is either painted or sprayed on to the inside of
the mould (So much for the tradition of laying the
keel; with a GRP boat the 'paint' is laid first!).
Unpigmented gelcoat looks and handles like a grey
translucent paint. Up to 10% Pigment paste is
added to give the required colour to the gelcoat. The
pigment is usually in liquid form and is thoroughly
mixed into the gelcoat to ensure a uniform colour.
Next, 1% to 3% of Catalyst (MEKP or Methyl Ethyl
Ketone Peroxide), is thoroughly stirred into the
pigmented gelcoat resin. With the catalyst added, the
gelcoat has to be applied quickly before it starts to
harden (cure). The length of time that it remains a
liquid is determined by the amount of catalyst added
(1% is slow, 3% is a fast mix.), and also by
temperature. The higher the temperature the faster the
cure. As a general guide 2% catalyst at 20ºC gives
15-20 minutes of 'pot life' (Pot life is the time
interval in which the resin can be used after the
catalyst is added and before the gelcoat starts to
harden).
Application
The catalysed gelcoat is either painted (using a paint
brush), or else sprayed (using specially formulated
gelcoat & a hi-tech compressed air sprayer) on to
the surface of the mould. The aim is to achieve a
uniform coating of about 0.5 mm thickness.
Curing
Once applied the gelcoat will start to cure (harden) in
about 15-20 minutes. The cure is 'air inhibited' which
means that the gelcoat next to the mould will cure
completely. The inner surface exposed to the air will
not cure completely until the lay-up resin (See Step
3), is added.
STEP 3 Adding the laminate
Catalysed lay-up resin is painted on to the tacky
gelcoat surface. A sheet of fibreglass fabric or
matting is stippled down on to the resin. More resin is
added to the sheet to ensure that it is completely
wetted out. Another layer of fabric or mat is added
followed by more resin. This process is repeated until
the laminate is of the required thickness. It is then
left to cure.
Choice of Fibreglass material
The most commonly used reinforcement is Chopped
Strand Mat (CSM). This consists of strands of
fibreglass held together by an emulsion or powder
binder to form a thin mat. Plain Weave Cloth (no
binder is necessary), gives a stronger more flexible
laminate but it is not so rigid as CSM. Both CSM &
plain weave cloth are usually supplied in rolls.
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A roll of Chopped Strand Mat (left) & (right) two rolls of Plain Weave Cloth tape
Lay-up resin
This is the base resin for the laminate. It looks like
a clear translucent paint and is easily applied by
brush or roller. It is activated (as is gelcoat resin)
by the addition of 2% Catalyst (MEKP or Methyl
Ethyl Ketone Peroxide), which gives a pot life of about
20 minutes at 20ºC. The catalyst is thoroughly
stirred into the resin immediately before use. Note
that unlike gelcoat resin, the lay-up resin curing
process is not air-inhibited. This results in a hard
plastic with no uncured resin surfaces.
Building up the Laminate
Catalysed lay-up resin is either brushed or applied by
roller on to a small area of the mould on top of the
tacky gelcoat resin. A sheet of chopped strand mat or
plain weave cloth is laid down on top of the resin and
stippled down using a stiff brush. More resin is added
on top and stippled into the sheet until no pale areas
remain. This means that the fibreglass is thoroughly
saturated ('wetted out') with resin. Another sheet of
fibreglass fabric (cut slightly smaller than the first
sheet so that there is an overlap between adjacent
sheets), is then added on top of the first sheet and
more resin brushed on until this second sheet is
'wetted out'. The two layers of fibreglass are then
forced together, expelling any trapped air, by means of
a metal hand (Consolidating) roller. Further sheets of
fibreglass and resin are added until the desired
thickness of the laminate has been achieved. This
process is repeated over the whole area of the mould.
What about the tacky gel coat layer?
The lay-up resin will bond to the tacky surface of the
gelcoat and at the same time exclude the air so that
the latent catalyst can complete the gelcoat cure. The
lay-up resin will cure at the same time ensuring a
strong bond between the two resins.
STEP 4 Completed hull
Once the resins have completely cured, the completed
hull is carefully separated from the mould. The
combination of the polished surface of the mould and
the use of a release agent usually makes the separation
a fairly easy process.
After separation, the surface of the gelcoat (which was in contact with the mould), should have a polished appearance. Final polishing and removal of any residual blemishes completes the process.
This section is to clarify the important differences between Gelcoat and a derivative called Flowcoat (also known as Topcoat). For most (but not all) gelcoat repair operations described in these pages, Flowcoat NOT Gelcoat must be used. However, both forms are used where several coats of gelcoat are painted on a large repair area.
Gelcoat is supplied either colourless or white. For other colours between 6-10% of an appropriate colour pigment is added to the colourless gelcoat and completely stirred in to ensure uniformity of colour. After the pigment has been added, between 1-3% of catalyst is thoroughly mixed with the pigmented gelcoat to enable it to cure. From a repair perspective the important characteristic of gelcoat is that the curing is 'air inhibited' which means that the surface exposed to the air will remain tacky (only partially cured). This is useful in ensuring good bonding where several coats of gelcoat are applied to a repair. However, the final coat should be Flowcoat; a modified gelcoat which sets hard throughout (see below).
To enable a complete gelcoat cure, 2% of a wax solution (wax dissolved in Styrene) is added which converts it to a Flowcoat (Topcoat). This can be pigmented and catalysed in exactly the same way as is gelcoat but (after application), the wax floats to the surface of the resin and excludes the air. This ensures that the resin cures to a hard plastic throughout. Unlike gelcoat, the resulting surface has a matt (unpolished) surface but it can be sanded and polished in just the same way to blende in perfectly with surrounding original gelcoat. From a repair perspective, multiple coats of flowcoat cannot be applied as the contained wax will prevent bonding between successive coats. Ordinary gelcoat is applied instead but is always capped with a final coat of Flowcoat. This ensures a uniform cure which can be sanded & polished.
Small quantities of pigmented flowcoat (marketed as Gelcoat Repair Kits), can be obtained from Stewart Brown at Churchouse Boats. For larger quantities it is cheaper to purchase from a chandler or dealer such as East Coast Fibreglass Supplies. Note that the product is either colourless (needing pigments to add colour), or white. The minimum quantity is a 1 Kg pack. Alternatively a 500g pack of gelcoat resin and a separate 100g pack of wax solution can be ordered. Add 10ml of the wax solution to the gelcoat (mix it very thoroughly), to convert it to flowcoat.
Shelf life
The shelf life of unpigmented flowcoat is guaranteed
(by East Coast Fibreglass Supplies), for 3 months but they
suggest a shelf life of 8-12 months if stored in cool
dark conditions. Storing it in the metal tin in which
it was supplied, I have used flowcoat which is in
excess of 2 years old with no problems. It degrades by
gradually curing to a hard plastic.
Following are some important guidelines to be followed when using fibreglass materials.
All polyester resins contain styrene. Styrene is very hazardous to health and should never come into contact with the skin. Gloves should be worn at all times. Styrene is a carcinogen and warrants the utmost care. Always be sure to minimise the amount of fumes inhaled and always try and wear a respirator, especially in confined spaces.
Like styrene, the GRP catalyst Methyl Ethyl Ketone Peroxide (MEKP) is very hazardous to humans. It is also a potent carcinogen. It is imperative that gloves be worn at all times when handling this stuff. Again, always avoid breathing MEKP fumes and wear a respirator, especially in confined spaces.
Acetone is a very strong solvent that dissolves all of the natural oils in the skin. Excessive exposure can lead to many forms of skin irritation. Acetone can also cause permanent skin damage, so handle it carefully.
Gelcoat should be handled with care to avoid skin contact. It is also very messy and difficult to clean up if spilt. As with many of the other materials, it’s important to avoid breathing the styrene fumes.