Heel dents or cracks are much less common these days with the advent of foot pads on all boards. In the old days(before pads were standard) it was obvious if a crack had started. These days you may not even know you have them until you see water dripping from the bottom edge of the pad. On slalom boards they usually only occur on the back foot heel area.
The last board I made had some cracks appear. It was not because the layup was too light, it was just sailed a lot in chop. I have lots of offcuts of the materials used in board manufacture so I could keep the repair light. If you dont have access to these you could fill the foam region with polyurethane foam but some have a lower than required density and can crush more easily than the eps core.
I use the West Systems 105 epoxy. You can buy a pump pack that dispenses the exact ratio of resin and hardener to take the guess work out of mixing the correct ratio.
Here is how I went about repairing the damage.
Remove the pads with minimum damage. This can usually be started from a curved or pointed corner. If the pad starts to tear you may need to slice the area that is not peeling with a sharp blade to help it along. If the dent or crack only affects a small area under the pad you may only need to peel the local area of pad away.
Now that you have cleared the pad away from the affected are proceed the press the deck around the crack or dent to identify where it has softened. Mark with a pencil the border of the affected area. This is the area you wil be removing as the material is damaged and needs replacing. With a sharp chisel you should be able to carefully shave the laminate away. Work from the pencil line in and try to angle the cuts so that the laminate on the edge will have a large bonding area.
The pvc sandwich foam should now be exposed. I then use a dremel tool to grind away all of the damaged pvc and eps core foam in a smooth concave shape.
With some tracing paper copy the outline of the pvc foam and transfer the shape on to a cardboard template. Cutout the template and copy it in pvc foam.
Do the same again only this time copy the eps foam outline onto a piece of eps foam.
Cut out each of the pieces of foam to the outline with a snap off knife. Now you need to sand the eps foam so that it neatly fills the void in the eps. It is not a problem if you go too far, it is a problem if it sits high though as this will be reducing the amount of pvc foam you can apply. Now with the eps sitting in place do the same with the pvc foam and fill the void. You do not need to get the top of the foam sitting below the skin as we will sand this down after the foams are cast into place. Use 6mm foam as you will have to sand it down to blend with the board. If you use 3mm you don't have as much freedom and the fit of the foam cutouts needs to be more exact.
Mix up some resing with microbaloons to a cream consistency, that is with minimum sag. Butter up the polystyrene block and the hole it is going to fill. Place the block in the hole and push it in so that it is level or just below the line of the existing eps foam. Now spread the excess filler (that is squeezing out around the block) over the top of the block and the edge of the pvc foam.
Pre-wet the glass patches with pure resin and place them on the block making sure that the edges of the glass finish on the pvc.
Now butter up the back of the pvc blocks and push them into place just enough so that filler is squeezed out evenly beneath the block. Tape the block into place starting from the center and working out with vertical strips. Leave the resin to cure overnight.
Once the resin has cured remove the tape and fair the foam with the body of the board. Work with a block and coarse paper of about 40 grit until nearly flush and then go down to 80 grit. A surform is the best tool to use if you can get your hands on one.
With the foam flushed off cut out a glass patch slightly bigger in size than the outline of the pvc foam. Cut out about 6 patches or so for heal repairs with each patch slightly larger than the last to feather the edge of the final laminate. Mask of the area outside of the largest patch to stop resin running over the board.
Mix up some resin and a small amount of microbaloons to butter up the pvc before laying up the patches. Allow to cure and then sand back smooth in preperation for a final primer and topcoat. If the layup has low points that wont be filled by paint fill them with microbaloons and sand flush.
Paint the repair with primer and then top coat to match the colour of the board. Polish to blend the top coat and re-apply the pads(and non-skid if required) using contact adhesive such as selly's quik grip.