How to repair a hole under a board or a surfboard
Tuto proposed by RORO (Sète)

The condition of the board (after a fall in a wave and a big knee blow on the hull) :

  

Preparation of the "Dremel", delimitation of the area with paint tape :

Cut the contour of the skin with the Dremel and a cut-off wheel (everything that has been deformed):

Remove the skin (it comes off quite easily after cutting)

    

We can see the depression to be filled: 

Another little stroke with the Dremel and the "grinding wheel" tool to make a first very progressive scarf (not sure if this phase is really necessary)

Prepare and pour the foaming epoxy resin (the magic product to have in your repair kit)

  

Wait for inflation + drying 24h

  

Sanding, first with the disc machine to go faster (be careful not to remove too much material)

Fine sanding with an electric sander

New tape + sanding of the scarf

Checking the height with a rigid ruler laid across (I see the day, it's a good sign!):
 
  

Preparation of the fabric (2 or 3 layers, 200g), the necessary to fill the day under the rule

Application of epoxy resin + fabric. A little on the board without the fabric, then a little between each fabric, just enough to completely impregnate each piece
 
  

Wait for complete drying (a little less than 24 hours should be enough at 20°)

Sand (electric sander) up to the protective tape (the fabric not impregnated on the sides will go away by itself when sanding, do not pull on it)

Manual sanding with water. Coarse grain (180) then fine (500). The result is not bad:

   

Polyester putty + fine sanding :
(Since I make my putty by incorporating MixFill 10 to my epoxy resin Surf Clear because the putty ployester is detached during shocks).

  

Paint in several thin layers (every 10 minutes)

  

End of the repair (varnish if available)