I have been casting kiteboard rails with PU for over 7 years and never had a board or rail fail yet.
I have kept my process real quiet so the big bully boy companies don't steal the ideas that I have built up but now I am I will share some of it with you
The stuff I use rated Shore 90 and cures in 5 mins with a white cured finish.
I make up an outline router template, this needs to be13mm smaller that the finished outline + rail.
Turn the bottom of the board so that is facing downwards.
Lay the template onto the corecell and position, using 2 small felt rooming names through the foot strap position marking points to secure to the corecell.
Set up the router with a 12.5mm bit and set the depth to that of the finished rail, for me this is 4 mm ish.
Route around the template leaving one continuous trench.
Place wide masking tape on the board side of the rail, keep it real close to the rail, this is to catch any overspill and make it easier to clean off later.
Now make sure the corecell is on a 100% level surface with some weight on it to hold flat.
Pour the PU into the trench and make sure you overfill the trench as PU shrinks.
The good bit here is that the PU gets into the open cells created around the trench as it was cut into the slot, the PU gets into these and locks solid to the core.
Once the Pu has fully cured, about 10 mins for the stuff i use, take a new Stanley knife blade and using it as a scraper flatten the overfill/overspill PU until its flat with the surface and remove the protective tape.
Cut around the outside of the cured PU rail and remove the unneeded corecel and you are left with outlined core blank with rail attached.
Turn the board over and shape the top down to the rail and then the to the tips,
Now fit the inserts and route some holes for the fin blocks and using the same PU set them in place.
TOP TIP - before you laminate over the core, take some thin "pin striping" tape the same width as your rail thickness and put it around the outside of the PU rail.
TOP TIP - Make this tape as bright as you can horrid lumi green works a treat, the purpose is to make it much easier to locate the rail when you are cutting or grinding the flash of the finished board. The tape help you identify if you going too deep or are a long way off the rail
Hope this help you guys out?
Unfortunately for me I can't build boards anymore or go Kitesurfing, the sports of Wakeboarding and Kitesurfing that I love so much has f00ked my knees, so why not share a little