You have to let the resin tack up to semi-solid before cutting the lap with a razor blade. A good test is to touch the resin on the board with your finger and if it's sticky but does not stick to your finger an pull a string of resin then it's ready to cut. You can also let the resin fully cure and cut through, it's just requires more force to cut through the solid cured resin. Much easier to cut when it's in a softer state.