Repairing a fiberglass monofin: lessons learned
Supplies for the repair job

Repairing a fiberglass monofin: lessons learned

This post was submitted by Luca Malaguti, a freediver based in Vancouver, Canada.

A few months ago I was diving with my fiberglass monofin, looking for crabs near the shore. My fin got stuck between two rocks in a narrow passage.  To my dismay, a little bit of torque was enough to create a 10cm fracture in my monofin (see the picture below). Not only was my monofin fractured, but the location of the crack was so  unfortunate that with each undulation of the fin the fracture would inevitably propagate further.

repairing fiberglass monofin
See here the crack in my fiberglass monofin. The area around it is already sanded

My fiberglass monofin:

Here’s a little history about my monofin. I purchased it from www.freedivershop.com, and had it (expensively) shipped to Vancouver, BC after having to pay even more in import fees. I bought the ‘Sport’ fin from the brand Leaderfins. Once it arrived, I noticed that both foot pockets were labeled “left”. On top of that the monofin was not symmetrical: one half was longer than the other by a few centimeters. As a starting fin it does the job, but beware of purchasing the cheapest products on the market. This fin was obviously improperly manufactured (the fracture itself was unrelated to its manufacturing).

Equipment for the repair job:

Fibertek, a fiberglass dealer in Vancouver, BC (http://www.fibertek.ca/) specializes in all fiberglass types and glues. I bought the toughest, and most heavily woven type of fiberglass (their medium weight cloth yard), and a special type of epoxy glue (the 250 mL epoxy resin for wet & cold curing and 125mL epoxy hardener both from Fiber Craft Aqua-Set). I used them according to the instructions (a 2:1 ratio) while keeping in mind temperature and humidity effects on setting time. In order to  repair a monofin, you will also need a stirring cup, a measuring cup, stirring sticks and a variety of sand paper (I suggest 80, 200 and 400 grit). You should also have nylon gloves and a mask, a wooden table board and a sharp knife (or better yet an X-Acto knife or even a scalpel to cut the fiberglass).

fiberglass monofin
Supplies for the repair job

The fix:

I sanded the spot around the fracture using the 80 grit sand paper around the crack. I then cut the fiberglass cloth with scissors but quickly learned this is a bad idea! Use a sharp knife instead on a flat wood surface to get nice clean edges, otherwise you’ll destroy the woven sheets and make a mess everywhere!

After cutting the fiberglass into strips (starting at 3cm x 5cm), I laid them perpendicular to the fracture and saturated them with the mixed epoxy. They have to be soaked but beware of putting on too much epoxy!

Subsequently I layered the fiberglass on top of each other over the damaged zone and allowed the epoxy to cure (2 – 3 days in a dry area).  Once everything is dry, you can use the 200 and then 400 grit sandpaper to smoothen the fiberglass. If you wish to paint it, make sure you use a paint specifically for epoxy and intended for marine activities.

Hard Lesson Learned:

Here’s the important part and what I learnt from my mistakes.

Patience is a virtue when repairing monofins. I should have been much more patient with fixing it: setting only a few layers at a time, allowing them to dry, testing the rigidity and then continuing with one layer at the time. The rigidity of a monofin can make or break your dive.

Secondly, pressure! I never thought of the epoxy as a chemical reaction but or course it is, and so if the layers aren’t tightly packed there will be some gas/air bubbles forming in between them. This will cause the fiberglass to crack or separate once force is applied. You really shouldn’t underestimate the amount of force generated while kicking underwater! I found this video, obviously after I made the mistakes, (https://www.youtube.com/watch?v=UlbglAhIHus) showing a method in drying the fiberglass epoxy using a “non-stick” polyethylene plastic, foam pads and weights.

To sum up the last two paragraphs: you should add layer by layer patiently. Allow them to dry, in the right environment, each time under heavy pressure. Failing to do so will result in improper settling of the fiberglass layers.

On the right hand side of the fin, the fiberglass/epoxy adhered properly to the fin’s natural fiberglass (picture below). On the other side it, the force of undulating the monofin was strong enough to rip the edge’s off, and then with water intrusion it just detached all the layers. Properly sanding down the surface before can go a long way; it will help the epoxy adhere to the smooth surface (which I forgot to do on the left side).

To be continued…

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Luca Malaguti

Luca Malaguti is a former engineer turned freediving professional athlete and founded Sea to Sky Freediving. He lives in Vancouver, Canada among other places including Dahab, Dominica and Philippines.

This Post Has 4 Comments

  1. John

    Great post and very helpful tips. One of my scuba fins is cracked – not a monofin but i’m sure this will work well for me.

    Thank again and great timing 🙂

    1. Jaap

      Good luck with the fix John, let us know how it goes!

  2. Gilbo Baggins

    This was 2016 but hopefully still be reliable process to follow with regards to new products coming out.How did it turned out after you done the repair?was it still the same performance wise?thanks for the reply in advance.

    1. Luca Malaguti

      Hey, turned out okay, but I recommend using a vacuum system to remove any air and apply tons of pressure to ensure proper bonding between fibres and resin!

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