The finishing topcoat I have applied has cracked and water is leaking in

You will tend to get cracks in the finishing topcoat if you apply the material too thickly, check the application rate for more information.

Unfortunately, the only way to fix the problem is to re-laminate over the cracked areas being careful to abrade the area prior to application.

The finishing topcoat should be applied at around half a litre per square metre and should be no thicker than 0.5mm.