Thermal Survey Types of Layups Wet Layups  During the thermal survey process, try to determine possible  hot and cold areas in the repair zone. Temporarily attach a  patch of the same material and thickness, several thermal  couples, heating blanket, and a vacuum bag to the repair  area. Heat the area and, after the temperature is stabilized,  record the thermocouple temperatures. Add insulation if the  temperature of the thermocouple varies more than 10 degrees  from average. The areas with a stringer and rib indicate a  lower temperature than the middle of the patch because they  act as a heat sink. Add insulation to these areas to increase  the temperature.  [Figure 7-43]  During the wet layup process, a dry fabric is impregnated with  a resin. Mix the resin system just before making the repair.  Lay out the repair plies on a piece of fabric and impregnate  the fabric with the resin. After the fabric is impregnated,  cut the repair plies, stack in the correct ply orientation, and  vacuum bag. Wet layup repairs are often used with fiberglass  for nonstructural applications. Carbon and Kevlar$^{®}$ dry fabric  could also be used with a wet layup resin system. Many resin  systems used with wet layup cure at room temperature, are  easy to accomplish, and the materials can be stored at room  temperature for long period of times. The disadvantage of  room temperature wet layup is that it does not restore the  strength and durability of the original structure and parts  that were cured at 250 °F or 350 °F during manufacturing.  Some wet layup resins use an elevated temperature cure and  have improved properties. In general, wet layup properties  are less than properties of prepreg material. Additional insulation can be placed over the repair area.  This insulation can also be extended beyond the repair area  to minimize heat being conducted away. Breather materials  and fiberglass cloths work well, either on top of the vacuum  bag or within the vacuum bag or on the accessible backside  of the structure. Place more insulation over cool spots and  less insulation over hot spots. If access is available to the  backside of the repair area, additional heat blankets could be  placed there to heat the repair area more evenly. Epoxy resins may require refrigeration until they are used.  This prevents the aging of the epoxy. The label on the  container states the correct storage temperature for each  component. The typical storage temperature is between 40 °F  and 80 °F for most epoxy resins. Some resin systems require  storage below 40 °F. Solutions to Heat Sink Problems Figure 7-43.  Thermal survey example. Insulate due to rib heat sink 300 °F Temperature Dwell Constant-watt-  density heat blanket Bonded stringer Patch perimeter Rib 240° 250° 2-INCH MIN 2-INCH MIN 200° 200° 280° 290° 300° 260° 7-26