Sanding your Hydrofoil

In this Blog I will tell you how to sand your Foil to prevent air ventilations during your session. If you follow these steps carefully you shouldn't have any problems with air on your wings and your Foil will be faster. If you have a slow Foil you might never have some flow demolitions because of sucked air. But as soon as you go faster and love to push a little bit harder you might have this problem. From experience I know that an impact with 50 km/h isn't as smooth as landing on wadding. 


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Before you start you need some tools to get a perfect result.  These are:

-Abrasive paper for wet sanding (500 or 800 and 2000)

- Sponge

- Sanding block

- Bucks with soft topping

- Water

- cloth

- Primer / Filler 

- Window cleaner

Step 1

The main problems of air ventilations are some small pinholes and scratches. These irregularities produces a vacuum and that creates air bubbles wich creates flow demolitions. 

So fill all the small pinholes on your mast, fuselage and wings. Don't put to much filler on the parts. It will get hard do sand it down smooth with out deforming the mast or the wing. 

I use a rubber glove and fill the holes by putting the filler with my index finger in the holes. After that I clean the excess filler with a paper towel. 

When everything is dry sand it down with the 800 paper. Use a sanding block and be super carefully. 

You can also spray the filler over the whole mast. Grind the surface with 500 before and clean it before spraying. The filler will hold better on the foil. 


Step 2

Now sand your mast from the leading edge to the tip with an angle of 40°. Sand always from the top to the bottom (use 2000)
If you sprayed the whole mast with filler use the sanding block tho get a smooth and flat surface. If you just had some small holes and cleaned them in step one, then use the sponge to sand. The sponge adapts to the surface. 

Sand always wet. Your abrasive paper will last longer and the water will bind the dust. Do it on both sides of the mast. 

Step 3

Use a masking tape to get a clean and parallel area to grind. This sanding with 500 parallel to the leading edge is very helpful for a clean water flow and prevents against flow demolitions. 

Use your index finger to grind between the masking tapes. Make this step also on both sides of the mast. 


This vertical sanding has the same result like vortex generators on airplanes. Just google "vortex generator" and you will see some pictures to get a better idea of them. 


Step 4

This is also a very important step for a smooth ride. At the trailing edge the parted water comes together and a wrong edge will creates a annoying whistling and creates water turbulences wich slows you down. 

Here I use the 500 paper and a hard wooden sanding block. I prefer to stick the abrasive paper with double-sided tape on the block to get the best result. Sand the edge with a 30° angle and break the sharp edge.

Step 5

Make the same steps now with your wings. On step three adapt the sanding area to the depth of the wing. It is important that you only sand step 3 on the upper side of the wing. Because just on this side the flow will tear. 

Sometimes the rear wing has an upside down profile. Then you have to sand the bottom side (respective the upper side of the profile)


When you have finished all these processes clean the foil with a wet cloth and let it dry out. To get the best performance clean the foil with window cleaner and don't touch it with your hands. Your hands always are a little greasy and at the points where you touched the flow will not be clean.

NOTE: I can't answer you directly on the Blog. So you won't get a massage when I answer. Just look in a few days again or write me a massage via contact sheet. 

Kommentar schreiben

Kommentare: 18
  • #1

    Marcel Goebels (Samstag, 16 Dezember 2017 01:44)

    Hey guys,
    Thanks a lot for the Tips �.
    Can you tell me which primer or filler you uses?

    LG Marcel

  • #2

    Riftboards (Sonntag, 17 Dezember 2017 22:55)

    Hi Marcel
    You can use different primers or fillers. The easiest way is using DUPLICOLOR Filler. Spray over the foil and sand it down. Spray thin layers! There are a lot of fillers on the market. Just google.
    The other way is using 2k filler (Polyester). Use a good squeegee to put it on the foil. Make sure that it is only in the pinholes! Otherwise you will have problems to sand it down.

    Lg, Simon

  • #3

    Leo F (Freitag, 16 Februar 2018 23:08)

    Thanks for these tips. Do you also apply a clear coat? Or just keep sanding the finish that came with the foil?

  • #4

    @Leo F (Samstag, 17 Februar 2018 21:30)

    You can easily sanding the finish of the foil. But you have to be careful that you did not sand through the varnish. If that happens, you can easy do a clear cote over it and sand it again.
    You don't have to sand much. Just sand it a little so that the varnish gets the structure of theabrasive paper.

  • #5

    Beni Kölin (Mittwoch, 21 Februar 2018 20:49)

    Hoi Marcel,

    wele Duplicolor Filler nimmsch genau? Filler isch grundierig gäl?
    Wot nüt falsches druf spraye...

  • #6

    Chris (Donnerstag, 22 Februar 2018 18:23)

    Thank you for posting on your blog about how to sand your hydrofoil. I have a question, as there is something not 100% clear to me.

    Steps #2 and #3 should be on BOTH sides of the mast/strut, correct?

  • #7

    @Chris (Donnerstag, 22 Februar 2018 22:29)

    Yes, both sides. I changed it in the text. Thank you

  • #8

    Jordi Peralta (Montag, 02 April 2018 13:46)

    Last one or two mili meters from the end of the trailing edge at the mast and also the wings on comet 2018. There is some wider part at he long if all mast and wings. This part is not constant from the profile of the mast and wings. Should I sand on it to do it smoth. This shape is like an arrow at the end of the trailing edge. Thanks

  • #9

    John Price (Montag, 25 Juni 2018 14:50)


    Thanks for the info! Any chance you could make a video showing these steps? I appreciate your efforts to help other foilers.

  • #10

    Garry (Montag, 02 Juli 2018 18:39)

    On step 3, do you sand just the one strip or do you repeat along the entire depth of the mast? Similarly for the wing. Thanks!

  • #11

    @Garry (Dienstag, 03 Juli 2018 16:44)

    Hello Garry
    You just have to sand the strip. After sanding and riding a lot of different foils we noticed that it depends on the profile of the mast if you need this strip or not. Try it first without the strip. The new Moses Coment Mast don't need this strip.
    If it works it will be fine, if you still have problems then make the strip.
    If you have a bad Mast you can do what ever you want and always have troubles.

    Hope I could help, Simon

  • #12

    Walter (Freitag, 12 Oktober 2018 11:32)

    Hi, thank you very much for your instructions! I'm trying zu remove flow demolitions from a 93cm Moses mast. I understood that the 1cm-strip step 3 must be on both sides of the mast. But only at the leading edge, not at the trailing edge, right? Andere there is nothing special to sand at the surface from the trailing edge to the tip except step 4 which I understood ist only at the first 2-5mm, correct?

  • #13

    @walter (Samstag, 13 Oktober 2018 12:35)

    It has to be only on the leading edge. Normaly you dont need the vortex generator stripes. Try it first without and if you still have ventilation you can do it with them. The trailing edge has to be sharp. Sand it with a 30 degree angle and break the supersharp edge.
    If you sand the surface with 40 degree or small circles does not make a difference.
    Also important are the wings. If they have some scratches you will have ventilation too. And always clean your foil woth windowcleander bevore you use it and never touch it with your hands bevore you go in the water. exspecially suncream is a killer and you will have ventilations.

  • #14

    Sylvain (Montag, 02 September 2019 20:50)

    Hello letape 1 il faut poncer de l avant vers l arriere ou de haut en bas?

  • #15

    @Sylvain (Montag, 02 September 2019 21:02)

    Ce n'est pas grave. Je préfère d'avant en arrière.

  • #16

    Jonas Axelsson (Donnerstag, 13 Februar 2020 09:33)

    The salami foildeath.

  • #17

    Jonas Axelsson (Freitag, 14 Februar 2020 14:40)

    What filler/glase do you use?
    I can only find gray or red ones. Not so estetic.

  • #18

    @Jonas Axelsson (Sonntag, 16 Februar 2020 21:27)

    Hello Jonas
    I use Epoxy with Graphit Powder.