Fiber composites since 1985

Repair of GRP boats

GRP boats are extreme­ly robust and durable. Nev­er­the­less, dam­age can occur due to col­li­sions or rough moor­ing maneu­vers. In this case, good advice is expen­sive! But: Almost any dam­age can be repaired. So first deter­mine the degree of dam­age and then deter­mine the strat­e­gy for repair. Sim­ple repairs include fill­ing holes or repair­ing minor, super­fi­cial cracks.

These dam­ages can be repaired by any­one. Struc­tur­al dam­age, on the oth­er hand, requires rebuild­ing load-bear­ing struc­tures from both sides of the lam­i­nate. With all the addi­tion­al work, such as fill­ing, sand­ing and paint­ing, sev­er­al days of work can be involved here. As a rule, this is a job for professionals.

Before we get start­ed, a few thoughts on epoxy or poly­ester resin. Most ships have been built with poly­ester resin and thus repairs with poly­ester resin are quite rea­son­able and above all pos­si­ble. Equal­ly pos­si­ble here, of course, are epoxy resins as long as the poly­ester resin is ful­ly cured. On the oth­er hand, ships built with epoxy resin can only be repaired with poly­ester resin to a lim­it­ed extent.

The rea­son for this is the dif­fer­ent elon­ga­tion at break. Stan­dard poly­ester resins have an elon­ga­tion of 2–3%, which is sig­nif­i­cant­ly low­er than EP resins, which have an elon­ga­tion of 4–7%. If you repair with a stan­dard poly­ester, there will be cracks in the repaired area which can con­tin­ue into the pre­vi­ous­ly undam­aged lam­i­nate. Only spe­cial types such as some iso-NPG resins or vinyl ester resins will work here. If in doubt, of course, take an epoxy resin to repair an epoxy ship.

Basic properties of the hardened laminate

Poly­ester resin
Epoxy resin
Low­er elon­ga­tion at break
High­er elon­ga­tion at break
High UV and weath­er resistance
Yel­low­ing under UV influence
Stress­es, as shrink­age appears in sol­id (gelled) phase
Low stress, as shrink­age takes place in liq­uid phase


Poly­ester resin
Epoxy resin
Clear­ly per­cep­ti­ble odor of styrene
Almost odor­less
Fast hard­en­ing
Slow hard­en­ing
Can be over­lam­i­nat­ed for a long time
Before over­lam­i­nat­ing must be ground and cleaned

Reinforcing fibers

Poly­ester resin
Epoxy resin
Glass mats
Glass fab­rics
Glass fab­rics
Glass scrims
Glass scrims
Car­bon fiber walkways
Car­bon fiber scrim


Poly­ester resin
Epoxy resin
In com­bi­na­tion with glass mat extreme­ly cost-effective
Epoxy with fab­ric and scrim are quite expen­sive raw materials

1. Simple repairs of GRP boats

1.1 Small holes

In the sim­plest case, only a hole from a stripped screw needs to be filled. There are two basic cas­es here: 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 plugged on one side. This is done with a sim­ple plug.

Work­ing steps:

  1. Drill or mill out to load-bear­ing laminate
  2. Shank the hole edge with a coun­ter­sink bit
  3. In the case of through holes, use a plug. Then fill hole with fiber filler (see appendix)
  4. Smooth sur­face with Japan filler and allow fiber filler to harden
  5. After hard­en­ing, sand, fill and varnish.

1.2 Large holes

Large holes are treat­ed sim­i­lar­ly to small­er ones. Only the method of plug­ging looks different.

Work­ing steps:

  1. Milling out or sand­ing down to the load-bear­ing laminate.
  2. Sand­ing the back side around the hole (approx. 5cm wide)
  3. For small holes, bev­el the edge with coun­ter­sink, for large holes, bev­el the edge with sandpaper
  4. Soak glass fab­ric with resin/hardener mix­ture and place over the hole on the reverse side. Lam­i­nate to the edge with­out bubbles.
  5. After cur­ing, apply fiber filler and smooth.
    Sand, fill and paint fiber filler after com­plete cur­ing.
    Repairs to struc­tur­al components

The use of fiber filler is not rec­om­mend­ed for repairs to struc­tur­al com­po­nents, espe­cial­ly if the lam­i­nate is made of fab­ric or scrim. In this case, only the use of fabrics/laid scrims and pos­si­bly mats can be considered.

2. Repairs to structural components

2.1 Filling large, continuous holes in structural components

  1. Milling or grind­ing out the hole to the load-bear­ing laminate.
  2. Bev­el the lam­i­nate at a ratio of 1:10. A 10mm thick lam­i­nate is beveled to 100mm around the hole.
  3. Cut fab­ric and/or mats, cut­ting 2 lay­ers so that they are approx­i­mate­ly 5mm larg­er than the hole.
  4. Pre-soak the two largest fab­ric sec­tions with resin and place them on the hole from both sides. Press the lam­i­nate pieces well in the edge area and deaer­ate. Allow to cure.
  5. As soon as the lam­i­nate pieces are slight­ly hard­ened, build up the rest of the lam­i­nate. Start with the largest pieces. Final­ly, lam­i­nate the small­est pieces. Ven­ti­late each lay­er well.
  6. Allow the repair area to cure and then sand smooth. Fill and varnish.

2.2 Filling large, continuous holes in structural components

It is still quite easy to repair a hole that is acces­si­ble from both sides, but for dam­age that is only acces­si­ble from one side, a lit­tle trick must be used. In prin­ci­ple, you use a “plug”, but here it is rather a thin plate.

Work­ing steps

  1. Cut a lay­er of tis­sue about 10–15 mm larg­er than the hole diam­e­ter. Soak it with resin/hardener and let it cure.
  2. Pre­pare the hole as under 2.1
    Make a hole in the cen­ter of the cured lam­i­nate lay­er and insert a wire through it. To pre­vent the wire from slip­ping through, bend it over and fix it with some SciGrip.
  3. Apply Sci­Grip to the edge of the lam­i­nate piece, put the “plug” through the hole and pull with the wire against the inac­ces­si­ble back side. After the adhe­sive has cured, pinch off the wire.
  4. Now the hole can be filled with lam­i­nate as already described in 2.1.

2.3 Filling large, continuous holes in sandwich laminates

Basi­cal­ly, the pro­ce­dure here is the same as for the pre­vi­ous dam­aged areas. Unlike before, only the sand­wich has to be restored here, so some foam or bal­sa wood has to be used. This is done with a light sand­wich adhe­sive. Then the lam­i­nate is built up.

Work­ing steps

  1. Milling or grind­ing out the hole to the sand­wich. Again, make sure to cham­fer the lam­i­nate at a ratio of 1:10.
  2. Cut out dam­aged sand­wich with hole saw.
  3. Cut out a piece from a sheet of (orig­i­nal) core mate­r­i­al with hole saw.
  4. Insert the “plug” with a lit­tle sand­wich glue. Wipe off escap­ing glue.
  5. Cut pieces of glass cloth and/or glass mat so that the largest is about 4–5mm larg­er than the ground out area. The small­est should cor­re­spond approx­i­mate­ly to the diam­e­ter at the base of the hole (i.e. at the sand­wich core).
  6. After the glue has cured, you can now lam­i­nate. Start again with the largest piece.
  7. Allow to cure and then sand the sur­face smooth. Fill and paint.

3. Osmosis removal

3.1 What is osmosis?

Reparatur Osmose
Osmo­sis dam­age in advanced stage

Osmo­sis dam­age is usu­al­ly due to poor work­man­ship and/or old age of the ship. The cause is usu­al­ly air bub­bles in the lam­i­nate (poor ven­ti­la­tion dur­ing pro­duc­tion) or in the gel­coat. Due to a con­cen­tra­tion gra­di­ent, these cause the water to dif­fuse through the plas­tic into the bub­ble. 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 pic­ture 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 caus­es more and more water to enter the bub­ble. Such dam­age is not eas­i­ly elim­i­nat­ed. Depend­ing on the sever­i­ty of the “infes­ta­tion”, a pro­fes­sion­al may be required. Nev­er­the­less, we would like to describe the basic pro­ce­dure of a restoration.

3.2 Restoration of osmosis damage

In the sim­plest case, a few bub­bles are found in the gel­coat. In the worst case, the entire hull is cov­ered with osmo­sis bub­bles, which are also locat­ed in the lam­i­nate. The first thing to do is to deter­mine where the osmo­sis bub­bles are locat­ed. To do this, prick sev­er­al bubbles

As a rule, 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 like vine­gar or even like almonds. In both cas­es, these are 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 some­what less. The gel­coat is sand­ed down to the lam­i­nate, fol­lowed by the buildup of a pro­tec­tive layer.

Work­ing steps:

  1. Sand­ing the gel­coat down to the laminate
  2. Check lam­i­nate for mois­ture con­tent (should be below 15%). If the mois­ture con­tent is too high, the hull must be dried.
  3. Apply a lay­er of lam­i­nate (160 g/m2 fab­ric with AMPRO resin) and let it cure.
  4. Fill­ing of the hull with S‑FAir 600, fol­lowed by sanding.
  5. Seal with Hempels Light Primer (5–6 layers)
  6. Build up new under­wa­ter coating

Case 2:
If the lam­i­nate is dam­aged, there is a lot of work to be done. In this case, not only must the gel­coat be removed, but the lam­i­nate must be stripped down to the sound mate­r­i­al. Then the lam­i­nate has to be rebuilt.

Work­ing steps:

  1. Removal of the gel­coat and the dam­aged lam­i­nate. As a rule, the hull is blast­ed in such cas­es. Sand­ing or plan­ing dam­ages too much healthy material.
  2. Mea­sure hull mois­ture. If more than 15%, dry the hull.
    Depend­ing on the lam­i­nate lay­ers removed, sev­er­al lay­ers of lam­i­nate must be built up. For this pur­pose, the blast­ed hull is filled. Allow to cure.
  3. Lam­i­nate build-up (at least 1 lay­er of glass mat or glass fab­ric). Allow to cure.
  4. Fill­ing and sand­ing of the surface.
  5. Apply the sealant.

This brief descrip­tion may give the wrong impres­sion that this is a fair­ly sim­ple task. For those who want to per­form this task them­selves, please note the following:

Sand­ing 1 m² of osmo­sis dam­aged lam­i­nate means about 1–2 hours of work (if the lam­i­nate is dam­aged only about 2mm deep). So for a 30‘ ship between 15 and 40 hours. The sub­se­quent dry­ing process takes sev­er­al weeks (much less in an appro­pri­ate anneal­ing cham­ber). Fill­ing and sand­ing again take about 15 — 40 hours (the inex­pe­ri­enced must usu­al­ly fill and sand much more often until the result is convincing).

Then the lam­i­nate is rebuilt, which takes about 5–20 hours. It fol­lows again spat­u­la and sand­ing with anoth­er 10–20 hours and the final appli­ca­tion of a sealant. Depend­ing on the mate­r­i­al between 2 and 6 hours. This quick­ly adds up to 100 hours of work or more, which means about 3–4 months of work for the hob­by­ist. Mind you, 2–3 hours of work every day after the actu­al work.

Dry­ing a hull usu­al­ly requires the use of anneal­ing cham­bers or at least heat blown. Both are rather expen­sive options. The rec­om­men­da­tion here can there­fore only be to have this work done by a pro­fes­sion­al. We will be hap­py to put you in touch with a lead­ing ren­o­va­tion com­pa­ny. If you still want to do the ren­o­va­tion your­self, please con­tact us. We will work out a con­cept for your ship togeth­er with you.

4. Necessary materials

All nec­es­sary mate­ri­als can be found in our online store. If you have any ques­tions or sug­ges­tions, please con­tact us!

Poly­ester resin
Epoxy resin
Mois­ture meter Skip­per: An indis­pens­able device when ren­o­vat­ing a ship. The device offers 3 dif­fer­ent mea­sur­ing ranges and deter­mines the mois­ture con­tent via 2 soft rub­ber electrodes.
Mois­ture meter Skip­per: An indis­pens­able device when ren­o­vat­ing a ship. The device offers 3 dif­fer­ent mea­sur­ing ranges and deter­mines the mois­ture con­tent via 2 soft rub­ber electrodes.
Fiber filler: To fill holes or deep dam­age in non-load bear­ing lam­i­nates, fill the AMPRO lam­i­nat­ing resin with glass beads and Col­loidal Sil­i­ca. Add 3% Col­loidal Sil­i­ca to the AMRPROTM lam­i­nat­ing resin/hardener mix­ture. Stir in well. Glass beads are then stirred in until a creamy, firm and eas­i­ly spat­u­laable mass is obtained. The put­ty must be applied with­in about 15 minutes.
Fine filler S/Fair 600: Alter­na­tive­ly, a ready-made filler can be used. The S/Fair 600 can be applied but­tery and eas­i­ly sand­ed. The max­i­mum lay­er thick­ness is 35 mm.
VE679P lam­i­nat­ing resin: VE lam­i­nat­ing resin for lam­i­nat­ing in under­wa­ter and above-water appli­ca­tions. 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 weath­er­ing and water resis­tance. Post-cur­ing should be car­ried out at a min­i­mum of 60°C for one hour per mm of lam­i­nate applied.
Ampro lam­i­nat­ing resin: EP lam­i­nat­ing resin for all repair work in the over­wa­ter and under­wa­ter hull. The resin is char­ac­ter­ized by good fiber impreg­na­tion and no for­ma­tion of amine blush. With three dif­fer­ent hard­en­ers, pro­cess­ing times between 35 min and 4 hours are pos­si­ble. Post-cur­ing is rec­om­mend­ed to achieve the best pos­si­ble weath­er and water resis­tance. It is urgent­ly required when using the slow and extreme­ly slow hardeners.
Hempel Light Primer: Light Primer is applied as the last coat before apply­ing an antifoul­ing. This can be done with a roller or with a spray gun (add thin­ner up to 20%). Apply­ing two coats (about 100 g/m2) gives an excel­lent adhe­sion primer for the antifoul­ing. If 5–6 coats (350–400 g/m2) are applied, a bar­ri­er lay­er is obtained to pre­vent re-osmosis.
Hempel Light Primer: Light Primer is applied as the last coat before apply­ing an antifoul­ing. This can be done with a roller or with a spray gun (add thin­ner up to 20%). Apply­ing two coats (about 100 g/m2) gives an excel­lent adhe­sion primer for the antifoul­ing. If 5–6 coats (350–400 g/m2) are applied, a bar­ri­er lay­er is obtained to pre­vent re-osmosis.


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