Hi Dsully, As I'm sure you know, the best option is actually to pour subsequent layers when the previous has reached the 'B' or 'Tack' stage of cure whereby the resin feels tacky to the touch but, when wearing gloves, no residue is lifted off on your fingertips, however, if this stage is missed then you need to wait for full cure and once fully hardened, abrade the surface with a 120-240grit abrasive paper, remove any dust and then pour the subsequent layer. The new layer will infill all of the surface scratches left in the keying process to again give a seamless bond.