Glue guns come in low-temperature and high-temperature (hot-melt) versions. Low-temperature glue guns operate at approximately 250 °F (121 °C) and are well suited when high temperatures are undesirable, such as gluing lace and cloth. High-temperature guns operate at approximately 380 °F (193 °C) and produce a stronger bond. Dual guns have a switch for both low- and high-temperature use. In addition to bonding surfaces together, hot-melt glue can be used to fill gaps, but the properties that allow gap-filling (high viscosity, high toughness, and so on) keep it from forming an adhesive film as thin and smooth as is possible with other adhesives. (For example, a wood joint properly made with hide glue may be invisible, marked only by a difference in grain at the seam line.) Bonds must be made quickly before the glue has time to harden. Usually it must be applied accurately with the glue gun, as it can not easily be spread, but it is always possible at any time to melt and spread the glue with a heat gun or a household clothes iron, which helps when bonding larger areas. Surprisingly, hot-melt glue can be used to assemble and repair foam models as an alternative to foam-safe Cyanoacrylate or UHU POR adhesive. Due to the insulating properties of the foam the hot-melt glue remains sticky for much longer than when used on wood, metal or plastics. Another development of glue gun technology allows the user to 'spray' an area with a semi-pressure sensitive hot melt adhesive. Using compressed air to force the adhesive at a controlled rate through a specially developed glue gun nozzle, a spiral pattern of adhesive is applied. The tackiness of the adhesive / open time is limited to just a few minutes, after which the adhesive will lose its tack, so coverage is limited.