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Whenever you remove a teak deck from a yacht there is a reason for it. Usually since it is old and leaky. you hope that the water is just under the teak but not yet in the sandwich structure of the deck itself. But the moment you do not think about it the bad things happen....

This article is about how to repair a formerly "teak-deck-covered" hatch by removing rotten core from inner wooden layer of the sandwich hatch. You can use this step-by-step guide of course for each kind of delamination repair somewhere on the deck or wherever you have a GFK sandwich structure.

During our project work of the projects #4 Remove 30-year old teak deck and replace with KiwiGrip (-> more) we also of course had to work on the hatches. They have been covered with teak too so we had to work on them as well. Good thing is we were able to take our three hatches home and work without any pressure. So that is why you see me working almost the entire time in a carhouse or a cellar.

Photo: The corner with the rotten core. Not yet fully opened.


How it began

All went fine on removing the teak of the hatches. I have worked on all three hatches. Dust and noise and dirt - nothing unusual for this kind of boat project. At the final hatch during last teak removing activities I recognised that my my working gloves felt wet. I had no clue where it came from. But then when I came closer with my multitool to remove remaining glue from the teak the gelcoat / fibreglass layer in one corner of the hatch was yielding. And that was indeed not good as you can imagine.

So I took a closer look and found out following: Through the holes of the screws from one mounting water has entered into the core layer of the sandwich structure of the hatch. And even worse the central layer consists of wood. What I did then was to follow the path of the water. I cut all parts of the wood out that got in touch with water. Some of it already started to rot away. It is really making your skippers' heart bleeding.


The following pictures show how it all looks like. 

Photo: Closer look to the wet / rotten core


Photo: Piece of core. Looks awful right? You even can press water out of it.


Photo: Fully carved out. All wet parts out. The entire surrounding can now be dried out fully.


Photo: This is full picture of hatch. You see it is a small corner - around 15cm by 25cm of part to be relaminated. 



Note: This project is work in progress. You get updates regularly. A lot of details, pics and videos are already available.

  • Planning -> done
  • Purchase -> done
  • Execution -> done
  • Wrap Up -> done


The theory

Before we step into the topic we recommend to read our article about the entire deck removal. You find many information about why it is important to get rid of the no longer sealed deck and what significant damages it could could cause. You find the article here ->  [Project #4] Remove 30-year old teak deck and replace with KiwiGrip (Part 1)     


What is the plan? 

Rotten core of a sandwich GFK structure is a serious damage. It has to be fixed properly. The most important part is to rebuild the structure and and bring back the necessary stability of the entire GFK sandwich structure. For that all bad material has to be put out and the sandwich has to be restructured. The best material for the core as of now is a PVC hard foam which we will are going to use for the repair as well. The advantage of foam compared to wood. It does not rot. Of course the main goal is preventing water getting into the structure it is just better to keep stability by using foam.

In addition it is likely that new screws will be drilled into the hatch for attaching it the deck. So we will go with following approach of drilling the screws in:

  1. Draw the hole with a drill
  2. Fill the hole with epoxy
  3. Enter the screws and seal them with butyl
  4. Seal the hinge with butyl


Drawing: General sandwich structure of a GFK. This is the structure that has to rebuild on the damaged area.






The project


What is the goal of the project?

At the end of the project the damaged area is fully replaced by fresh core material properly laminated. The surface should then be ready to go on with applying the final deck coating or paint.


Main steps of the project

  1. Scratch out and remove the rotten parts that need to be replaced.
  2. Let the entire component dry to make sure remaining humidity will be out.
  3. Sand the inner parts and the edges properly.
  4. Cut a piece of core material that fits into the hole and prepare fibreglass mats..
  5. Fit the hatch to the workpiece, laminate the new core in and seal everything.


Estimated time


Purchase items / material to be used

Following components, tools etc. are needed:

  • Sanding machine
  • Sanding paper (grit: 80, 120)
  • Ventilation roller for laminating
  • Dremel to sand at areas where it is not easy to reach by hand or by sanding machine
  • Hard foam sheets thick (we need 1.5cm thickness. We take 5mm sheets and epoxy them together for additional stability)
  • Epoxy incl. hardener
  • Fiberglass mat (163 or 300 gram per square meter)
  • Brush for applying epoxy
  • Plastic cup for mixing epoxy
  • Acetone
  • Wooden pick for mixing epoxy
  • Scale for mixing epoxy with hardener
  • Microfiber or any other epoxy thickener for filling slightly larger holes
  • Mask (FFP2, FFP3 or "better")
  • Plastic foil
  • clamp for adhering the foam pieces
  • Gloves
  • Jigsaw (we use it for cutting the foam)



Feel free to download all document we provide to you. You can read it, use it, change it and use for your own individual purposes. Whatever makes sense for you. You are even allowed to use it on commercial level. No restrictions from SV LIMA side.


 Purchase and material list plus time planning (MS Excel, 20KB,  *.xlsx, password: S&V&LImA11&)


1. Scratch out and remove the rotten parts that need to be replaced.

The most important part is to put the humidity away from the core layer. Also all already rotten material has to removed as well. So be better generously and make a centimetre more than you think.

Remove gelcoat and fibreglass where necessary to reach the rotten areas

Scratch the material out from the sandwich structure

If there is some material that still looks good it might also be useful to drill some holes into the gelcoat to let these parts dry. You later can fill the holes with epoxy - no problem.


2. Let the entire component dry to make sure remaining humidity will be out.

It as simply as it sounds: It must be dry in the core layer. So really wait some days in a warm environment to let it fully dry. If you have a humidity measurement tool the value of the humidity should be below 18%.


3. Sand the inner parts and the edges properly.

Usually you have small corners and edges. You hardly get to that part with a standard sander. So consider using a Dremel and / or sanding paper you use by hand. Sand everything until old material is out and the sides are rough enough for a proper rebuild with epoxy, fibreglass and epoxy thickener (per example microfibre). 


4. Cut a piece of core material that fits into the hole and prepare fibreglass mats. 

Photo: Hard foam PVC sheets - 0,5mm of thickness


Preparing the workpiece

This is just a small piece but a bunch of work. In the video you can see all relevant steps for creating the workpiece made from 3 layers of PVC foam. This piece will be super stable and you will never ever have trouble with rotten material in this area.

Main steps if you do not want to watch the video:

  1. Draw the shape of the piece that needs to be replaced on a transparent paper.
  2. Cut the paper along the shapes.
  3. Apply the shape of the piece on the PVC sheet.
  4. Cut three pieces of the same size from the sheet.
  5. Sand the pieces.
  6. Glue the sheets together with epoxy for getting a 15mm thick workpiece.
  7. Let it sit for one day and remove overhanging epoxy.



5. Fit the hatch to the workpiece, laminate the new core in and seal everything.

This is the final part. The workpiece is ready. Now do following steps.

  1. Fit the workpiece into the shape of the hatch. Either cut the hatch itself to make the workpiece fit or do some final work on the piece itself to fit in. Both is valid and makes a smooth merge.
  2. Take a fibreglass mat (you can use 300g/m2 or 168g/m2, whatever fits in is ok) cut it to fit in the hole of the hatch.
  3. Prepare the hole: Sand the hole, then do a vacuum cleaning and then you need to clean with Acetone. 
  4. Mix epoxy and make a first layer directly in the hole.
  5. Apply the fibre glass mat onto the wet epoxy in the hole.
  6. Pour epoxy on the mat and make it fully soaked (Before you close the holes of the underlying GFK layer with a proper tape - below)
  7. Now you can paint the workpiece also with epoxy. Put the epoxy on the side of the workpiece that goes into the hole. Also paint the sides properly.
  8. Put the workpiece in the hole with slight pressure. A little of the epoxy should squeeze and fill the lower parts of the gaps.
  9. Fill the gaps between the workpiece and the edges with epoxy or thickened epoxy (thickened epoxy can be created with mixing epoxy with microfibre, microballons and many more things). Put the thick material in the gaps several times. It needs its time to get into. We used a spatula to put it down.
  10. As the gaps are full with epoxy it is time to fix the workpiece with some clamps. Let sit over night.
  11. When all is dried out your can sand the surface with 120 sanding paper.
  12. The damaged area is now completely rebuild, more stable than before and ready for applying either epoxy filler, gelcoat or whatever you want to put on your surface.


Photo: Fully set rebuild core with foam. Can now be used for painting or teak or whatever you want to apply.


The video

Here you can find the full step-by-step guide of "How to replace rotten wood from sandwich deck of a hatch".


Wrap Up / Dos and Don'ts

Frankly spoken. I was pretty annoyed that it happened. Repairing delamination is one of the worst pieces of work you can do on a boat. It is much work and it takes some time.

BUT: It is good practice. As I have never done it before I now simply had to do it. So this gives me a good opportunity to learn and also to lose the "fear" of such kind of difficult work. In the end it turned out that is was not so complicated. If you have an idea what needs to be done it is then only work to be done.

The DOS:

+ Make your plan and purchase all you need (full list above) before you start. You should have everything in place.

+ More is better: Removing damaged or rotten material completely and then if you think you are done remove another cm. Then make some holes in the surrounding area of the damage part. It helps to really fully drying.

+ Have a vacuum cleaner in place that exhausts the toxic dust - this is really critical material. So better collect it in a dust bag.

+ Wear a mask: You deal with toxic material. So go with the best mask you have. Either a full face mask or an FFP3 mask (what I did).

+ There is room for errors. The size of the workpiece does not fit? Sand it a bit or cut the hole the piece goes in a bit. The entire work we did produced such a strong lamination. This is by far more stable than the rest of the deck part you are working on. 



- Do not use ductape for closing the GFK layer underneath to prevent epoxy pouring down through potential holes. It does not really hold properly. Better use something like the blue 3M tape from Scotch. 



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