Photo: Silvio Hesse
Most hydrofoils make contact with the ground at some point in their life. It's almost unavoidable. I hardly notice small scratches myself. However, if you're participating in races or other competitions, it can be a different story.
Deeper scratches or chips can be more significant. It also depends on where the damage is located. On my front wing, the small chips at the tip were particularly bothering me. It was time for a repair.
In this post, I'll walk you through how I approached it and what mistakes you should avoid, step by step.
Repair Steps
The post consists of two parts:
The first part covers sanding the foil, in case small chips or scratches can simply be sanded away.
The second part shows the steps for rebuilding with epoxy, in case damage cannot be removed by sanding alone.
Quick tip upfront: If you only need a quick overview, you can scroll to the end of the post: there are some there ;). For everyone else, here's the more detailed version.
Sanding Steps
1. Damage Analysis
To choose the right approach, you should examine the damage closely.
Small damages can sometimes be removed relatively easily and quickly by sanding. For deeper scratches or chips, fillers are used before sanding.
In a video about sanding an Armstrong foil, which I found very helpful, I understood that damage deeper than 0.4 millimeters (especially in the central areas of the foil) should preferably be filled. To ensure you don't sand off too much material, it might even be advisable to choose an even smaller limit – especially if you don't know the exact material thicknesses of your foil.
Although the chips at the wingtip were larger, I initially decided to sand them away anyway, as I had to remove loose or crumbling material for the epoxy buildup.
2. Preparation for Sanding
Materials & Tools
Among other reasons, because you don't want epoxy and carbon dust in your lungs, wet sanding is recommended. Therefore, when choosing sandpaper or pads, make sure they are suitable for wet sanding. I used Bosch sandpaper, which I already had in the workshop. To be honest, I mostly tested if it was suitable for wet sanding: it held up and did the job well. Only with the 1200 grit sandpaper did I have the impression that there was a bit less "sand" on the paper at the end. That could also have been due to the water.
I used the sandpaper in the following grits:
80 grit: For roughly sanding down excess material during rebuilding
120 grit: For (re)shaping
320 grit: For smoothing and removing small scratches
600 grit: To make the foil smooth
1200 grit: Fine sanding 1
2000 grit: Fine sanding 2 (optional)
I didn't sand again with 2000 grit. But I'll definitely do it later.
For (re)shaping, you can probably start with a coarser 80 grit. However, I preferred to approach it slowly with 120 grit. This removes less material and requires more sanding than with 80 grit, but the potential errors are also smaller and it leaves fewer marks.
To properly control and guide the sandpaper, I used a sanding block made of cork and a foam sanding block for the coarser work. The foam sanding block offers the advantage that it adapts slightly to the shape of the foil and holds the sandpaper well thanks to the Velcro on the block. With the cork sanding block, I have a somewhat more direct feeling of power transfer.
To be able to wet sand continuously, I filled a bowl with water, in which I regularly dipped the sandpaper to keep it wet and clean it at the same time. I used kitchen paper to lightly clean the foil.
Photos: Silvio Hesse
Protection
In case I stirred up already dried epoxy, carbon, or fiberglass dust, I also used a respirator in addition to wet sanding.
I also recommend covering the floor with something that can be cleaned more easily than the floor itself or disposed of later. I didn't do that, but I wished I had when I was cleaning. ;)
Mounting
To work on the underside of the wing well, I assembled the mast, fuselage, and front wing and clamped everything together, upside down, in a small workbench. To avoid damaging the mast, I attached the mast cover and didn't tighten it too much.
To also have a good view of the top side when (re)shaping the tip, I also worked on the wing disassembled so that I could turn it over.
Photo: Silvio Hesse
3. Let's go: (Re)shaping by sanding away
To give the foil a nice hydrodynamic shape again, I first placed the 120-grit sandpaper on a cork sanding block so that I could hold it on the sides along with the block. After soaking everything with plenty of water, I started removing bothersome areas and working towards the new shape.
Video: Silvio Hesse
As I approached the new shape, I attached the sandpaper to the foam sanding block and continued working towards the desired new shape.
Photo: Silvio Hesse
When the new shape was almost achieved, I took the sandpaper in my bare hand and guided it along the curve, so that I could feel the shape of the foil with my fingers. This is especially recommended for the curve of the leading edge.
Photo: Silvio Hesse
With my fingertips, I repeatedly felt for small imperfections, in addition to the visual check.
During all sanding processes (including with the subsequent grits), I repeatedly dipped the sandpaper into the water to re-wet and clean it.
It is recommended to apply the new shape to the other foil tip as well. Therefore, the process can be repeated on the other wingtip. However, I would recommend only tackling that once you know exactly what the final, new shape looks like on the damaged side.
4. Sanding with 320-grit sandpaper
With 320-grit sandpaper, you change the shape much less. I mainly used it for finetuning and to remove small scratches around the area and on the rest of the wing.
I used the sandpaper with both the cork and foam sanding blocks, and sometimes in my bare hand.
I continuously felt for small chips, scratches, and unevenness with my fingertips and worked on them until the result literally felt good.
Video: Silvio Hesse
5. Smoothing with 600-grit sandpaper
Once everything is in the desired shape and smaller scratches are largely removed, the surface should be beautifully smooth again. For this, I used 600-grit sandpaper in the next step.
To have a uniform surface on the entire front wing later, I worked on the entire wing (i.e., surfaces, leading, and trailing edge).
Video: Silvio Hesse
On the surfaces, I performed the sanding motion in the direction of water flow on the foil. I don't know if it makes a difference, but I saw something similar in the video about sanding the Armstrong foil.
Video: Silvio Hesse
6. Fine sanding with 1200-grit sandpaper
To ensure the wing has a beautifully smooth surface and the water flows around it with almost no resistance, I then worked everything again with 1200-grit.
The result was already a very smooth, slick surface. But if you really want to go all out, you can also work everything again with 2000-grit sandpaper in the next step.
Does it really have to be exactly these grits?
No, you don't have to use exactly these grits. In the instructions I've seen, they were sometimes used or recommended, but also others. Even with the sanding tips I received from other foilers on the water, other grits were sometimes used in addition to those shown here. However, everything was roughly in similar sizes.
The jumps between grit sizes in the individual steps also do not have to be followed exactly. However, as far as I know from woodworking, it is important not to choose too large gaps between the grit sizes used and to really perform several sanding steps. Coarser grits remove more material, but also leave marks. The next finer grit removes less material and eliminates the marks of the previous grit. This works best if the gaps between the grit sizes are not too large, as grits that are too fine do not remove enough material to eliminate the marks of coarse grits. The gaps between the grit sizes chosen here are already quite large. So I would not choose larger gaps, but rather smaller ones if necessary. However, I was satisfied with the grit sizes used. The foil is nice and smooth and performs great again. It doesn't have to look like new for me.
Afterward, I also processed the stabilizer accordingly.
Once both wingtips are brought into the new shape and everything is smoothly sanded, the work would actually be finished. However, it had already become clear to me beforehand that the chips were too deep. I had to rebuild the area with additional material.
Steps for rebuilding with epoxy
As suspected, it turned out that the chips were too large to simply sand them away. I had to add a little material to properly repair the foil.
Materials & Tools
For the rebuild, I used epoxy. As far as I know, this is used for most high-quality foil wings.
Epoxy is used in larger quantities (e.g., for board construction or larger repairs) in two components, the epoxy resin and a corresponding hardener. Fillers may also be added. When repairing larger areas, epoxy is also used in combination with fabric layers (often carbon fiber fabric for foils and wingfoil boards and fiberglass fabric for surfboards). You can find more about this in our article on wingfoil board repair.
Because the area to be built up was very small, located at the tip, I only had to add very little epoxy, and it needed to be done quickly, I used Suncure, a fast-curing epoxy-fiberglass mixture from a tube that hardens under UV light. The fiberglass particles in the epoxy make it somewhat easier to work with and more stable. In addition to Suncure, there are other UV-curing epoxy mixtures in a tube (e.g., Solarez).
For application and spreading, I used a wooden spatula and my fingers. To protect my hands, I wore thin nitrile gloves. And to ensure the Suncure cured properly even with little sunlight, I also used a small UV lamp.
To prevent Suncure from getting on areas where it shouldn't be, or to protect the edge area from sanding marks, you can use painter's tape. This can be easily removed later.
Photo: Silvio Hesse
1. Prepare the area to be rebuilt
> Remove loose material, rough sand the area, and clean thoroughly
Loose material should be removed for better bonding. I used a cutter and sandpaper for this.
Photos: Silvio Hesse
The contact surface should generally be sanded with a coarser grit (e.g., 120) and thoroughly cleaned (remove dust, dirt, grease residue from handling, e.g., with an alcohol pad). I didn't do this properly the first time I applied the material due to time constraints. The Suncure held anyway, but after repeated, multiple, and quite hard impacts on Baltic Sea sandbanks, areas appeared that might later have led to the detachment of the added material. Before that happened during a session, I therefore preferred to remove the added epoxy myself beforehand and carried out the repair more conscientiously.
What I did wrong:
- I was in a hurry and hadn't sanded the area again with coarser sandpaper, but simply applied the Suncure to the surface previously sanded with 1200-grit sandpaper (nicely smooth). However, Suncure adheres poorly or not at all to this.
- I hadn't cleaned the surface thoroughly again.
- Clever as I was, I also applied the Suncure outdoors during the first repair, so the curing process started immediately when the epoxy was out of the tube. This gave me hardly any time to spread it properly, and I partially spread already hardened epoxy.
2. Material application
To prevent the Suncure from hardening immediately, I chose a place with low UV radiation for application during the second repair.
Before applying, I kneaded the tube lightly again to further mix the epoxy-fiberglass mixture.
I applied the Suncure directly from the tube and then spread it a bit with a wooden spatula and my fingers. In the area where I had to apply more material, I used transparent adhesive tape to hold the epoxy in place.
If you don't want the epoxy to get on the rest of the foil, you can apply painter's tape around the area to be repaired. This protects it and can be easily removed later.
As a precaution, I applied a bit more epoxy to ensure I wouldn't have any areas where material was missing later. However, you should make sure not to apply it too thickly so that the epoxy cures properly through UV radiation (if necessary, several thin layers are better). You should also keep in mind that anything you apply too much of will have to be sanded away again.
Photo: Silvio Hesse
3. Allow to cure under UV light
Suncure needs UV light to cure. So I placed the wing in the sun. Under ideal conditions, Suncure probably cures after just a few minutes.
However, since we don't necessarily have ideal conditions in Germany and cloud cover can reduce UV radiation so much that the curing process takes significantly longer, I also used a UV lamp. This also seems to work, but for larger areas, due to the smaller light cone (at least with my small UV lamp), it also takes a bit longer.
I mainly used the UV lamp as a supplement, e.g., after sunset or after I had sanded free small previously enclosed air bubbles during shaping, which then formed tiny "craters" and I therefore applied a second thin layer of epoxy again.
Photos: Silvio Hesse
Safety tip: Do not look directly into UV light.
4. Shaping and Smoothing
After the Suncure has cured (it took about an hour for me), it can be worked on in steps 1 - 6 with sandpaper - shaping and smoothing multiple times. This time, I also paid more attention to not leaving any "steps," especially at the transitions, but to make everything flush. Because I had quite a lot of excess material where the transparent tape was, I first sanded it down with 80-grit sandpaper.
Photos Tip from above and below: Silvio Hesse
For larger damage, it may be advisable or necessary to use an epoxy-carbon composite for repair, as well as possibly taking further measures. For minor damage, Suncure should be sufficient.
Long Story short: All steps in a quick overview
Steps for Sanding
1. Assess damage
> decide whether sanding down and (re)shaping is sufficient for the repair
2. Preparation
> Secure the wing for processing.
> Prepare sandpaper with grits 120, 320, 600, 1200, sanding block(s)
> Place a water-filled bowl for wet sanding near the wing
> If necessary, wear respiratory protection and protect the floor from contamination.
3. Sanding down and (Re)Shaping with 120-grit sandpaper
> first with a block, then partially with sandpaper in the bare hand to have more control and feel for the shape and curves
Adapt the other wingtip to the new shape of the repaired wingtip (if you are sure what the final new shape of the repaired side looks like).
4. Sanding with 320-grit:
> Finetuning, removing small scratches and chips. Use fingertips to feel for unevenness.
5. Smoothing with 600-grit sandpaper
6. Fine sanding with 1200-grit sandpaper
If necessary, then second fine sanding with 2000-grit
Steps for Rebuilding with Suncure
1. Prepare the surface to be built up
> Remove loose material, roughly sand the area and clean thoroughly
2. Material application
> in a place with low UV radiation
> spread with a wooden spatula and/or fingers
> if necessary, use transparent tape to hold thicker epoxy applications in place
> if necessary, use painter's tape to protect the adjacent area
3. Allow to cure under UV light
> Duration depends on the sun's position and cloud cover
> if necessary, use a UV lamp as a supplement
4. Shaping and Smoothing
> remove large material excesses with 80-grit sandpaper if necessary
> controlled shaping with 120-grit
> finetuning with 320-grit
> smoothing with 600-grit, 1200-grit, possibly 2000-grit
Small disclaimer: I am not a professional shaper. I am merely sharing my personal experience of how I repaired my damaged foil. The described procedure is based on research, tips from other foilers, as well as various instructions and videos that I applied to my specific case.
