Essentially, this is how it works… Inside of the mold, after the usual mold release wax is applied, the "gel coat", and "skin coat" of thin fiberglass reinforcement are applied in the conventional manner and allowed to cure. From here on everything differs. Next in the infusion process the parts outer skin of fiber reinforcement fabrics are carefully fitted into the mold over top of the skin coat. These are put in dry and held in place with a spray contact adhesive. Because the technicians are not hurried and concerned with the narrow resin curing period as would be in conventional lay up, attention can be paid to quality and the conscientious cutting, fitting and orientation of the fabrics fibers and core. Next, in the case of a cored part, the structural core materials are cut and fitted, and adhered into place. Then the inner skin of reinforcement fabrics is carefully fitted over the core to form a sandwich. Subsequently in the case of a boat's hull the longitudinal stringers are cut, fitted, and put into place, with their fiberglass fabrics fitted over top. The installation of these materials may take several days. Next the resin distribution hoses and vacuum lines are laid out atop the fiberglass and the entire inside of the mold is covered with a large sheet of loosely fitting plastic sheeting and sealed onto the mold’s perimeter. With a vacuum pump all the air in this "vacuum bag" is evacuated which compresses or de-bulks the dry stack of reinforcement fabrics. Through the series of feed hoses sealed into the bag, catalyzed resin is then sucked via the vacuum from large mixing containers. Over a couple hours the resin migrates throughout the mold saturating the entire stack of laminate. The vacuum is kept on until the resin has cured several hours later. The vacuum bag and feed hoses are removed, and the part's lamination is complete. 

The bottom line is the quality of an infused part is stronger, lighter and superior to a piece done using conventional hand lay-up because…

Traditional methods of bonding the core to the skin call for a polyester paste that is manually spread onto the cured surface of the fiberglass with the core being bedded into it. Clamping pressure is applied until the bonding material is cured. The integrity with this method relies upon the technician’s skill level, the performance of the bonding paste and its ability to adhere to the cured skin. This process can leave air voids within the bond layer and in the many “contour slits” in the core. This in conjunction with the inherent relative weakness of the bonding material all constitute strength losses.