(MF) or PF resin, is applied to the joint between successive boards and cured under end pressure using a continuous RF curing system. After the resins have cured, the lumber is planed on each side to ensure smooth surfaces for gluing. Once planed, a glue extruder spreads the resin onto the lumber. This resin is most often phenol-resorcinol-formaldehyde, but PF resin or melamine-urea-formaldehyde (MUF) resin can also be used. For straight beams, the resinated lumber is stacked in a specific lay-up pattern in a clamping bed so that a mechanical or hydraulic system can press the lumber together. For curved beams, the same process occurs, but lumber is instead stacked in a curved form. These beams are cured at room temperature for 5–16 hours before the pressure is released unless the hydraulic pressure is combined with RF curing which can reduce the length of time needed for curing. Once the glulam beams are completely cured, they are removed from the clamping system the wide-side faces are sanded or planed to remove any resin that is squeezed out between the boards. The narrow top and bottom faces may also be sanded if necessary, based on the desired appearance. The corners are often rounded as well. Specifications for appearance govern whether or not additional finishing is completed. Additional finishing may include filling knot holes with putty, sanding the beams further, and applying sealers, finishes, or primers. Technological developments Resin glues When glued laminated timber was introduced to the building technology in the early twentieth century, casein glues, which are waterproof but have low shear strength, were widely used. Joints with casein glues had detachment failures due to inherent stresses in the wood. The invention of cold-curing synthetic resin glues in 1928 ("Kaurit") and other urea-formaldehyde glues solved these problems - resin glues, which are inexpensive and easy to use, are waterproof and enable high adhesive strength. The development of resin glues contributed to the wide use of glued laminated timber construction. Finger joints The use of finger joints with glulam allowed for production of glulam beams and columns on large scale. Glulam finger joints were developed to provide broad surface area for gluing. Automatic finger-jointing machines help cut the finger joints, connect and glue them together under pressure, allowing for a strong, durable joint, capable of carrying high loads comparable to natural wood with the same cross-section. Computer numerical control Computer numerical