Repair of GRP

Repair of GRP boats and components | With videos at the end of the article


GRP boats are extremely robust and durable. Nevertheless, damage can occur due to collisions
or rough berthing maneuvers. In this case, good advice is expensive!
But: Almost any damage can be repaired. So you first determine the degree of damage and determine
then set the strategy for the repair. Simple repairs are, for example, filling holes or smaller,
remove superficial cracks. This damage can be repaired by anyone. With structural damage
on the other hand, load-bearing structures must be rebuilt from both sides of the laminate. With all the additional
Work such as filling, sanding and painting can take a few days to work here. This is usually the case
then work for professionals.
Before we get started, a few thoughts on epoxy resin or polyester resin. Most of the ships are
were built with polyester resin and therefore repairs with polyester resin make sense and above all
possible. Epoxy resins are of course also possible here as long as the polyester resin is fully cured. The other way round
Ships made of epoxy resin can only be repaired to a limited extent with polyester resin. The cause lies in the
different elongations at break. Standard polyester resins, at 2-3%, are significantly below EP resins, which is more
4-7% elongation at break. If you repair it with a standard polyester, there are then cracks in the
repaired area that can continue into the previously undamaged laminate. Here only work
special types such as some Iso-NPG resins or vinyl ester resins.

If in doubt, you can of course use an epoxy resin to repair an epoxy ship.

Polyester resin (UP)Epoxy resin (EP)
Basic properties of the hardened laminate
Lower elongation at breakHigher elongation at break
High UV and weather resistanceYellows under the influence of UV
Tensions because of shrinkage in the solid (gelled) phase
Low tension, as shrinkage takes place in the liquid phase
Clearly noticeable smell of styreneAlmost odorless
Fast curingSlow hardening
Can be laminated for a long timeThorough sanding and overlay is required before each overlay
cleaning required
Reinforcement fibers
Glass matsGlass fabric
Glass fabricGlass scrim
Glass scrimCarbon fiber fabric
Carbon fiber scrim
In combination with glass mat, it is extremely cost-effectiveEpoxy with fabric and scrim are quite expensive
raw materials
Repair of GRP 2
Repair of GRP

Technical advice
Repair of GRP boats and components

  1. Simple repairs

1.1 small holes
In the simplest case, only one hole needs to be filled from a torn screw. There are 2 basic ones
Cases: a through hole or a so-called blind hole.
The procedure is the same in both cases with one exception. A through hole must first be on one side
be locked. This is done with a simple plug.
Work steps:

  1. Drilling or milling to a stable laminate
  2. Sharpen the edge of the hole with a countersink drill
  3. Insert plugs for through holes. Then fill the hole with a fiber spatula (see appendix).
  4. Smooth the surface with a Japanese spatula and allow the fiber spatula to harden
  5. After hardening, sand, fill and paint.

1.2 Big holes
Large holes are treated similarly to smaller ones. Only the type of plug looks different.
Work steps:

  1. Milling or grinding to a stable laminate
  2. Sand the back around the hole (about 5cm wide)
  3. For small holes, bevel the edge with a countersink bit, for large holes the edge with sandpaper
  4. Soak glass fabric with resin / hardener mixture and place on the back over the hole. Bubble-free on the edge
  5. After hardening, apply a fiber spatula and smooth it out.
  6. Grind, fill and paint the fiber filler after it has completely hardened.
  7. Repairs to structural components
    When repairing load-bearing components, the use of fiber filler is prohibited, especially when the laminate is off
    Fabric or scrim. Here only the use of fabrics / scrims and possibly mats comes into question.

2.1 Filling of large, through holes in structural components

  1. Milling out or grinding out the hole until the laminate is stable
  2. Bevel the laminate in a ratio of 1:10. A 10mm thick laminate is made 100mm around the hole
  3. Cut the fabric and / or mats, cutting 2 layers so that they are about 5mm larger than the hole
  4. Soak the two largest fabric sections with resin and place them on the hole from both sides. in the
    Press the laminate pieces on the edge area well and ventilate. Let harden.
  5. As soon as the laminate pieces have hardened slightly, build up the remaining laminate. With the largest
    Pieces begin. Finally, laminate the smallest pieces. Ventilate each layer well.
  6. Allow the repair area to harden and then sand it smooth. Fill and paint.
Repair of GRP 3

2.2 Filling of large, through holes in structural components that are only accessible from one side
It is still quite easy to repair a hole that is accessible from both sides
fix damage accessible from one side with a little trick.
In principle, a “stopper” is used, only this one is more of a thin plate.

Work steps

  1. Cut a layer of tissue that is about 10-15mm larger than the hole diameter. With resin / hardener
    soak and let harden.
  2. Prepare the hole as under 2.1
  3. Make a hole in the middle of the hardened laminate layer and insert a wire through it. So that the
    Bend the wire and fix it with a little SciGrip.
  4. Apply SciGrip to the edge of the laminate piece, insert the “plug” through the hole and with the wire
    pull against the inaccessible rear. After the glue has hardened, pinch off the wire.
  5. Now, as in 2.1, the hole can be filled with laminate.

2.3 Filling of large, through holes in sandwich laminates
Basically, you proceed in the same way as with the previous damaged areas. In contrast to before, must
here only the sandwich is made again, so some foam or balsa wood has to be used. This
done with a light sandwich glue. Then the laminate is built up.

Work steps

  1. Milling out or grinding out the hole up to the sandwich. Make sure that the laminate is in the
    Chamfer ratio 1:10.
  2. Cut out the damaged sandwich with a hole saw.
  3. Cut a piece out of a sheet of (original) core material with a hole saw.
  4. Insert the “stopper” with a little sandwich glue. Wipe off any adhesive.
  5. Cut pieces of glass fabric and / or pieces of glass mat so that the largest is about 4-5mm larger than the
    sanded point. The smallest should be about the diameter at the bottom of the hole (i.e. at the sandwich core)
  6. After the glue has hardened, you can now laminate. You start again with the largest piece.
  7. Allow to harden and then sand the surface smooth. Fill and paint.
osmose 1
  1. Elimination of osmosis

3.1 What is osmosis?

Osmosis damage is mostly due to poor workmanship and / or
attributed to the ship’s great age. Triggers are mostly air bubbles in the laminate (poor ventilation during manufacture) or in the gelcoat. Due to a concentration gradient, these lead to the water diffusing through the plastic into the bladder. There is always “less” water in the plastic than in the surrounding sea or lake. Therefore, such a bubble will fill with water and continue to grow until it bursts. In the image on the right, the bubble would be on the right and the surrounding water (the sea) on the left.
The concentration gradient creates the osmotic pressure, which ultimately ensures that more and more water gets into the bladder.
Such damage cannot be easily removed. Depending on the severity of the “infestation”, a professional has to work here. Nevertheless, we want to describe the basic process of a renovation.

3.2 Restoration of osmosis damage
In the simplest case, there are a few small bubbles in the gelcoat. In the worst case, the entire
Hull littered with osmosis bubbles that are also in the laminate.
The first thing to do here is to determine where the osmosis bubbles are. To do this, one pierces several vesicles. In all
Usually water runs out. If the bubble is in the gelcoat, there are no glass fibers (case 1). If glass fibers are found, the laminate is damaged (case 2). The leaking water usually smells of vinegar or even almonds. In both cases it is a question of decomposition products of some components of the plastic. In any case, each of these bubbles must be dried well.


Case 1
If only the gelcoat is damaged, the effort is a little less. The gelcoat is sanded down to the laminate,
a protective layer is then built up.

Work steps

  1. Sanding off the gelcoat down to the laminate
  2. Check the laminate for moisture content (should be below 15%). If the moisture content is too high, it must
    the hull to be dried.
  3. Apply a layer of laminate (165g / m2 fabric with AM21 or SP320 resin). Let harden.
  4. Filling the fuselage, then sanding
  5. Seal with Eposeal 300 or HPE Primer
  6. Apply new underwater paint

If the laminate is damaged, there is a lot of work to do. In this case, not only must the gelcoat be removed,
but the laminate has to be removed down to the healthy material. Then the laminate is rebuilt.

Work steps

  1. Remove the gelcoat and the damaged laminate. In such cases, the trunk is usually blasted.
    Grinding or planing damages too much healthy material.
  2. Measure body moisture. If the hull is more than 15%, let it dry.
  3. Depending on the removed laminate layers, several layers of laminate must be built up. To do this, the
    blasted hull puttied. Let harden.
  4. Laminate structure (at least 1 layer of glass mat or glass fabric). Let harden.
  5. Filling and sanding the surface.
  6. Apply sealing.

This brief description could give the wrong impression that this is a fairly simple task.
For those who want to solve this task themselves, the following should be noted:

Sanding 1m2 of osmosis-damaged laminate means about 1-2 hours of work (provided the laminate is only about 2mm deep
is damaged). For a 30 ‘ship, between 15 and 40 hours. The subsequent drying process takes
several weeks to complete (significantly less in a corresponding tempering chamber). Fill and sand
take another 15 – 40 hours to complete (the inexperienced usually has to fill in significantly more frequently
and grind until the result is convincing). Then the laminate is rebuilt, which takes about 5-20 hours in
Claims. This is followed by filling and sanding for another 10-20 hours and the final application
from a seal. Depending on the material, between 2 and 6 hours. So quickly 100 working hours or
more together, which means around 3-4 months of work for the hobbyist. Mind you, 2-3 hours every day
Work after the real work.
The drying of a hull usually requires the use of tempering chambers or at least heating fans.
Both a rather expensive option.
The recommendation here can only be to have this work carried out by a professional. We mediate you
would like to get in touch with a leading renovation company. Should you still consider the renovation yourself
Want to take a hand, talk to us. We work with you to develop a concept for your ship.

Necessary materials

Polyester resinEpoxy resin
Skipper moisture meter
An indispensable tool when renovating a ship. The device offers 3 different measuring ranges and determines the moisture content using 2 soft rubber electrodes.
Skipper moisture meter
An indispensable tool when renovating a ship. The device offers 3 different measuring ranges and determines the moisture content using 2 soft rubber electrodes.
Fiber filler FSP-HV 1538
Excellent glue and spatula. Highly thixotropic, large layers can also be produced. Easily smoothable. Hardening with standard MEKP, maximum 3% addition. Can also be used to glue in structural components.
Fiber spatula
To fill holes or deep damage in non-load-bearing laminates, the laminating resin is filled with glass spheres and colloidal silica. 3% Colloidal Silica is added to the laminating resin / hardener mixture (for example Ampreg 21 or 26). Stir in well. Then glass balls are stirred in until a creamy, firm and easy to trowel compound is created.
The spatula must be used within about 15 minutes
Fine filler FSP HV-1528Fine filler S / Fair 600
Alternatively, you can use a finished spatula.
The S / Fair 600 is buttery and easy to apply
grind. The maximum layer thickness is 35 mm.
Laminating resin VE679PA
VE lamination resin for lamination in underwater and above water areas. Thixotropic VE / DCPD system with excellent impregnation properties. Gel time around 35 minutes. Lloyd’s approval. Post-curing is recommended to achieve the best possible weather and water resistance. Post-curing should take place at at least 60 ° C for one hour per mm of applied laminate.
Laminating resin Ampreg 21
EP laminating resin for all repair work in both surface and underwater hulls. The resin is characterized by good fiber impregnation and a low tendency to develop amine reddening. With four different hardeners, processing times between 35 minutes and 4 hours are possible. Hardener fast: 35min Hardener standard: 50min Hardener slow: 100min Hardener extremely slow: 4 hours To achieve the best possible weather and water resistance, post-curing is recommended. When using the slow and extremely slow hardeners, it is imperative. For post-curing, 50 ° C for 16 hours is recommended.
Topcoat 2208X
Iso-NPG Topcoat, the ideal top coat before the usual underwater painting.
HPE epoxy primer
The last layer before applying an antifouling is the HPE primer. This can be done with a roller or with a spray gun (add up to 20% thinner).
If you apply one layer (about 340 g / m2) you get an excellent primer for the antifouling. If you apply 3-5 layers, you get a barrier layer to prevent renewed osmosis.
Repair of GRP 4

Even larger damage in the GRP area can be easily repaired with these instructions.

WEST SYSTEM 105-K Fiberglass Boat Repair Kit

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