I found with the stuff I use and which will remain nameless, sorry guys.
As the PU cures it leaves a slightly damp residue, its really important to remove that and the 40 grit then provides a mechanical key for the epoxy, glass to bond to.
Done 10 + boards with this PU and its amazin, not one delam or cracks.
I use a fast curing PU (60 seconds), it has a shore hardness of 90.
Its really hard to the touch but flexes more than enough for any board.
I tested the PU.
This stuff will shatter if hit hard with a hammer, it can be lethal.
However once encased in glass, it simply scuffs and does not seem to crack, chip or shatter despite the hammer test results
PU Noobs - Tip time
So you have your core upside down on a perfectly flat table.
The core has had a 4 or 6mm deep x 12,5mm wide Rabbit as the guys in the US call it or channel to the rest of the world ready to take the PU.
Tape the core to the table.
Mask around the inside of the channel leaving 2mm gap to the inner channel edge.
Pour the PU and remember to allow a little extra for shrinkage as it goes off.
Once you see it starting to harden, to the point where your finger nail wont leave a mark, grab a Stanley knife blade, hold it sharp edge onto the PU at an angle of almost 90 degrees.
Now use it as a hand scraper and shave off the excess PU until level to the core surface.
The 2mm gap will give a little extra bonding of PU to Core for the shaping process later.
Once the PU is fully cured, cut near to the outer edge of the poured rail and remove the excess corecell.
Sand back to the PU rail edge and make as smooth as a babies bottom.
Using some "bright" but thin masking tape, electrical tape or something like car pin strip tape, put this around the entire parameter of the finished "visible" side of the rail, then Lam up the top and bottom on your rocker table.
The idea of the tape is that it helps you when cutting the lam flash off and in the final sanding down as you know when you hit the rail as you hit that tape first indicating STOP, it also keeps the epoxy off the rail edge.
My finished rail edged are profiled later on to remove the sharp edges which avoids chipping even more.
I PU all the fixtures, from foot strap inserts to fin blocks.
I have made my fin block by drilling 12mm holes through the finished and shaped core, one hole for each fin screw and locator.
Using tape, block the holes from the shaped core side of the core.
Turn the core over and level the tips flat with packing.
Pour the PU and do the knife blade trick to level.
I have found Blocks like this allow the tips to flex better than one long block and the tape trick means no additional sanding.
Hope this helps.