Repacing Rotten Deck Core, Part II

10 Feb

After a couple weeks of being distracted by other projects the deck core seemed quite dry so I went ahead and put it all back together. This was actually the first fiberglass work I’ve ever done. I was working alone with just just a couple books for helpers and to my great surprise the repair turned out to be quite easy, even fun, and the deck feels solid as a rock. It was a very satisfying experience to fix a problem seemingly so serious.

As things were drying out on the deck I read the West System Fiberglass Repair Manual cover to cover a couple times. I can’t recommend this book enough, it’s a minutely detailed guide on most any epoxy repair you might do on a boat, and it’s free. (Find it here, usually. For some reason the link seems to be down at this moment.) They also have a searchable website with a ton of detailed repair info, including this guide to replacing deck core Fiberglass Deck Repair which I used extensively.

Using the 15oz fabric from that guide as a ballpark I ordered a roll of quite heavy bi-axial fiberglass cloth and a yard of end grain balsa. Then we just laid the balsa in a bed of thickened epoxy, faired it a little with more epoxy and covered this with a few ;layers of cloth. It was really that simple to repair, although we still haven’t quite finished the process of making it blend in with the rest off the deck!

To start we beveled the edge of our repair area. The proper ratio is a 12:1 bevel, meaning the width is 12 times the thickness of the fiberglass but with a thick deck like mine you can fudge it a good bit. Where I had room I did about 3″ for my 1/4″ deck skin.
Then I wet out the area with neat epoxy, glooped in a bunch of 
really thick structural filler and pushed the balsa into it. For good measure I wet the balsa out too.
The idea here is to fill all gaps with thickened epoxy. A key to strong fiberglass work is consistency in structure so it is important to avoid filling large gaps with epoxy. The thickened epoxy is harder and more brittle than fiberglass or balsa wood and can create stress points which wont flex with the rest of the material.
I weighted the whole thing down and let it set.
The next day it looked solid with very few air bubbles so after a quick sand to help the new layers key in we (yes, finally helpers!) put more thickened epoxy on top and began to lay up the fiberglass.
To determine how many layers we needed I had earlier laid up a couple 1″ squares with various numbers of layers. Once trimmed for a sharp edge these gave me a good measurement for the thickness of the fiberglass I had bought. I decided to use five layers for a 1/4″ top skin which was one too many. Fairing, (filling in gaps to create a smooth surface) is much easier than sanding down fiberglass and I learned in this and subsequent projects that it is usually better to use one too few layers of glass rather than one too many.

Our newly laid up deck. The entire process to this point took little more than twenty hours work and less than $100 in material though it was spread over a few weeks with many hours of reading and planning in between.

In keeping with West System’s advice we laid up the fiberglass with the largest layers on the bottom and smallest on top. The theory behind this is that if you sand down high spots you will be sanding the smallest layers and leaving the more structurally important large layers unmolested. The thick mat gave me a very rough edge which required lots of sanding with a belt sander. If I were doing this again along with less layers I would top things with a couple layers of light fiberglass cloth to help blend the edges. The end product would be the same but this would save hours of finishing work.

After plenty of sanding we were ready to fair the surface with thickened epoxy using a lightweight, sandable filler. Though easier than sanding down high spots I found fairing much more difficult than other fiberglass work. I certainly wasn’t helped by the size and shape of my repair, nor that I was trying to correct significant warping in the bottom skin. When there are significant variations in your surface fairing requires slowly build up to smooth, getting a little closer each time.

 I had trouble getting a consistency I liked with the fairing filler. When too runny it was hard to control and when too thick it would leave gaps like these. I had a much easier time when I gave up on trying to get a smooth finish with each layer and instead focused on just getting a little better each time.
 Then it’s back to sanding down high spots. In the end it worked out well though I don’t feel like I ever developed an effective system.

And there you have it. Not such a difficult repair after all. But then again, it’s a rare occasion when things on a boat go so much according to plan.

This article was syndicated from Safe At Harbour But Meant For The Sea: DIY Sailing with Paul Calder


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