Photo: Silvio Hesse
Most hydrofoils make ground contact at some point in their lives. This is almost unavoidable. Small scratches hardly affect the ride, at least at my riding level. If you participate 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 on the tip were particularly troublesome. It was time for a repair.
In this article, you will learn step-by-step how I proceeded and which mistakes you should avoid.
The Repair Steps
The article first shows the steps for sanding the foil and then the steps for rebuilding with epoxy, in case chips cannot be removed merely by sanding.
A quick tip: If you only need a brief overview, you can scroll to the end of the article: there is one ;) For everyone else, here is the slightly more detailed version.
Sanding Steps
1. Damage Analysis
To choose the right approach, you should carefully examine the damage.
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 that you don't sand off too much material, it might even be advisable to choose an even smaller threshold – especially if you don't know the exact material thickness of your foil.
Although the chips on the wingtip were larger, I initially decided to sand them off anyway, as I had to remove loose or crumbling material for the epoxy build-up.
2. Preparation for Sanding
Materials & Tools
Among other reasons, because you don't want epoxy and carbon dust in your lungs, it is advisable to wet sand. Therefore, when choosing sandpaper or pads, make sure they are suitable for wet sanding. I used Bosch sandpaper that I already had in the workshop. To be honest, I mostly tested whether 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 little less "grit" on the paper at the end. That could also have been due to the water.
I used the following sandpaper 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 sand the foil smoothly
1200-grit: Fine sanding 1
2000-grit: Fine sanding 2 (optional)
I didn't sand again with 2000-grit. But I will certainly do it later.
For (re)shaping, you can probably start with a coarser 80-grit. However, I preferred to work slowly with 120-grit. This removes less material and requires more sanding than with 80-grit, but the potential for errors is also lower, and it leaves fewer marks.
To properly control and guide the sandpaper for coarser work, I used a sanding block made of cork and a foam sanding block. The foam sanding block offers the advantage of conforming slightly to the shape of the foil, and the velcro on the block holds the sandpaper well. With the cork sanding block, I had a somewhat more direct feeling of power transmission.
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 paper towels for light cleaning of the foil.
Photos: Silvio Hesse
Protection
In case I stirred up already dried epoxy, carbon, or fiberglass dust, I also used a respiratory mask 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 while cleaning. ;)
Mounting
To properly work on the underside of the wing, I assembled the mast, fuselage, and front wing, and clamped everything together, upside down, into a small workbench. To avoid damaging the mast, I attached the mast cover and did not tighten it too much.
To get 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 Down
To give the foil a nice hydrodynamic shape again, I first placed the 120-grit sandpaper on a cork sanding block so I could hold it on the sides along with the block. After thoroughly wetting everything, I started removing bothersome areas and working towards the new shape.
Video: Silvio Hesse
Once I had approached the new shape, I clamped the sandpaper onto 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 curved along the rounded edge, so I could feel the shape of the foil with my fingers. This is particularly recommended for the curvature of the leading edge.
Photo: Silvio Hesse
With my fingertips, I constantly felt for small imperfections in addition to the visual check.
During all sanding processes (including with the subsequent grits), I repeatedly dipped the sandpaper in water to re-moisten 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 tackling this only when you know exactly what the final, new shape looks like on the damaged side.
4. Sanding with 320-grit
With 320-grit sandpaper, you change the shape much less. I mainly used it for fine-tuning and to remove small scratches around the area and on the rest of the wing.
I used the sandpaper sometimes with the cork and foam sanding blocks and sometimes in my bare hand.
Continuously, I 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 become nice and smooth again. For this, I used 600-grit sandpaper in the next step.
To have a consistent 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 helps, but I had seen something similar in the video about sanding the Armstrong foil.
Video: Silvio Hesse
6. Fine Sanding with 1200-grit Sandpaper
To give the wing a beautifully smooth surface and allow water to flow 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 process everything again with 2000-grit sandpaper in the next step.
Do I really need to use 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, besides those shown here, other grits were sometimes used. However, everything was roughly in similar sizes.
Also, the jumps between grit sizes in the individual steps do not have to be followed exactly. However, as far as I know from woodworking, it is important not to choose the gaps between the used grit sizes too large and to really perform several sanding steps. Coarser grit sizes remove more material but also leave marks. With the next finer grit size, less material is removed, and the marks of the previous grit size are eliminated. This works best if the gaps between the grit sizes are not too large, as too fine grit sizes do not remove enough material to eliminate the marks of coarse grit sizes. The gaps between the grit sizes chosen here are already quite large. So I would not choose the gaps larger, but possibly smaller. 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 again for me.
Afterwards, I also treated the stabilizer accordingly.
If both wingtips are reshaped and everything is nicely sanded smooth, the work would actually be finished. However, it had 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 be simply sanded away. I had to add a little material to properly repair the foil.
Materials & Tools
I used epoxy for the rebuilding. 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 major repairs) in two components: epoxy resin and a corresponding hardener. Fillers may also be added. When repairing larger areas, epoxy is also used in conjunction with fabric layers (often carbon fiber fabric for foils and wing foil boards, and fiberglass fabric for surfboards for wave riding). 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 needed to add very little epoxy, and it had to be quick, I used Suncure, a fast-curing epoxy-fiberglass mixture in a tube that hardens under UV light. The fiberglass particles in the epoxy make it somewhat easier to work with and more stable. Besides Suncure, there are other UV-curing epoxy mixtures in a tube (e.g., Solarez).
For application, 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.
Photo: Silvio Hesse
1. Prepare the Surface to be Built Up
> 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 area should generally be sanded with a coarser grit (e.g., 120) and thoroughly cleaned (remove dust, dirt, grease residues from handling, e.g., with an alcohol pad). I didn't do this properly the first time due to time constraints. The Suncure still held, but after repeated, multiple, and quite hard impacts on Baltic Sea sandbanks, areas appeared that could potentially have led to the detachment of the added material later. Before that happened during a session, I preferred to remove the added epoxy myself beforehand and carry out the repair more conscientiously.
What I did wrong:
- I was in a hurry and did not sand the area again with coarser sandpaper, but simply applied the Suncure to the surface previously sanded smooth with 1200-grit sandpaper. However, the Suncure adheres poorly to not at all to that.
- 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 left me hardly any time to spread it properly, and I sometimes spread already partially hardened epoxy.
2. Material Application
To prevent the Suncure from hardening immediately, I chose a location with low UV radiation for the second repair when applying it.
Before applying, I kneaded the tube lightly again to mix the epoxy-fiberglass mixture a bit more.
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 and can be easily removed later.
I applied a bit more epoxy as a precaution to ensure that I wouldn't have any areas with missing material later. However, you should be careful not to apply it too thickly so that the epoxy cures properly under UV radiation (if necessary, apply several thin layers). Also, keep in mind that anything you apply too much of will have to be sanded away later.
Photo: Silvio Hesse
3. Allow to cure under UV light
Suncure requires UV light to cure. For this, I placed the wing in the sun. Under ideal conditions, Suncure reportedly cures in 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 little longer.
I mainly used the UV lamp as a supplement, for example, after sunset or after I had sanded off small, previously trapped air bubbles during shaping, which then formed tiny "craters" and I therefore applied a second thin layer of epoxy.
Photos: Silvio Hesse
Safety tip: Do not look directly into the UV light.
4. Shaping and smoothing
After the Suncure has cured (it took about an hour for me), it can be worked on with sandpaper in steps 1 - 6 - i.e., shaped and smoothed multiple times. This time, I also paid more attention to not leaving any "steps" at the transitions, but to make everything flush. Because I had a lot of excess material at the spot where the transparent adhesive tape was, I first sanded it down with 80-grit sandpaper.
Photos tip from top and bottom: Silvio Hesse
For larger damages, it may be useful or necessary to use an epoxy-carbon composite for repair and, if necessary, take further measures. For smaller damages, Suncure should be sufficient.
Long Story Short: All steps in a brief overview
Steps for sanding
1. Assess damage
> decide if sanding down and (re)shaping is sufficient for the repair
2. Preparation
> secure the wing for processing.
> lay out sandpaper in grits 120, 320, 600, 1200, sanding block or blocks
> place a bowl filled with water 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 partly with sandpaper in your bare hand to have more control and feel for the shape and curves
Adjust the other wingtip to the new shape of the repaired wingtip (if you are sure what the final new shape of the repaired side will look 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 location 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 adjacent areas where Suncure should not land
3. Allow to cure under UV light
> Duration depends on sun position and cloud cover
> if necessary, use a UV lamp as a supplement
4. Shaping and smoothing
> remove large excess material with 80-grit sandpaper if necessary
> controlled shaping with 120-grit
> finetuning with 320-grit
> smoothing with 600-grit, 1200-grit, and if necessary 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, and various instructions and videos that I applied to my specific case.
