Where a board is constructed using a sandwich construction a repair must rebuild the damaged layers. Typically there is a outer layer of reinforcements (glass / carbon etc), a sandwich layer of Divicell foam, another layer of reinforcements and finally the inner core.
In the following example, the rail of the starboard has been cracked where the heel sits on the front foot strap – probably from a hard landing.
The outer broken layers need to be removed so the inner layers can be repaired. A simple patch on the outside will surely fail again as all the stress will be concentrated on the repair.
In the case the damage is under a padz, so this is removed first. The area is sanded until the core is reached. Note how there is a gently chamfered edge to allow a nice scarf joint to be formed later.
The core needs to dry the core out at this stage if the board has been wet. An effective way is to apply a vacuum to draw the moisture out.
The inner eps core need to be inspected for damage and repaired if necessary. Use 2 part polyurethane foam for this and once cured fair with the inner skin.
Rebuild laminate structure by replacing each layer. First skim the area with epoxy filled with micro balloons, this fills the surface of the foam with lightweight filler, rather than lots of resin to the area. Now replace the inner skin with same weight reinforcements the surrounding material. A micrometer can be used to help estimate the weight of the orignal cloth - 1mm is approximately 1000gms/m2 cloth, so 0.2mm is 200gms/m2 cloth.
The divicell can now be replaced at the same time. I used two 5mm thick pieces bonded together with epoxy/glass bubbles. This is now vacuum bagged to compress the laminate together. There is no real other way to attach sheets of foam without risking voids.
The divicell is now faired so it is the thickness of the outer laminate below the surrounding finished area. Use a straight edge to ensure there are no high spots and avoid low spots. It is better to be slightly low, than high.
The outer skin is now replaced. Like the inner skin the aim is to replace what was there before so hard spots are not introduced. I add a couple of extra layers of glass over carbon to help feathering in later. When the resin is added, leave the edge dry to ease feathering into the surrounding area. I also mask the existing non slip and paint to keep the resin off it.
The outer skin is vacuum bagged down to compress the laminate together and removes excess resin.
Once the resin has cured, remove the vacuum bagging consumables and fair into the surrounding area. Onto the paint stage.