not be used if it has exceeded its shelf life and/or has become  thick or jelled.  Mechanical shaking is recommended for all coatings within  5 days of use. After opening, a test with a hand stirrer should  be made to ensure that all the pigment has been brought into  suspension. Mechanical stirring is recommended for all two- part coatings. When mixing any two-part paint, the catalyst/ activator should always be added to the base or pigmented  component. The technical or material data sheet of the coating  manufacturer should be followed for recommended times of  induction (the time necessary for the catalyst to react with  the base prior to application). Some coatings do not require  any induction time after mixing, and others need 30 minutes  of reaction time before being applied.  Thinning of the coating material should follow the  recommendations of the manufacturer. The degree of  thinning depends on the method of application. For spray  application, the type of equipment, air pressure, and  atmospheric conditions guide the selection and mixing  ratios for the thinners. Because of the importance of accurate  thinning to the finished product, use a viscosity measuring  (flow) cup. Material thinned using this method is the correct  viscosity for the best application results. Thin all coating materials and mix in containers separate from  the paint cup or pot. Then, filter the material through a paint  strainer recommended for the type coating you are spraying  as you pour it into the cup or supply pot. To obtain the correct spray pattern, set the recommended air  pressure on the gun, usually 40 to 50 psi for a conventional  gun. Test the pattern of the gun by spraying a piece of  masking paper taped to the wall. Hold the gun square to the  wall approximately 8 to 10 inches from the surface. (With  hand spread, it is the distance from the tip of the thumb to  the tip of the little finger.)  All spray guns (regardless of brand name) have the same type  of adjustments. The upper control knob proportions the air  flow, adjusting the spray pattern of the gun.  [Figure 8-11] The lower knob adjusts the fluid passing the needle, which in  turn controls the amount or volume of paint being delivered  through the gun. Pull the trigger lever fully back. Move the gun across the  paper, and alternately adjust between the two knobs to obtain  a spray fan of paint that is wet from top to bottom (somewhat  like the pattern at dial 10.) Turning in (to the right) on the  lower, or fluid knob, reduces the amount of paint going  through the gun. Turning out increases the volume of paint.  Turning out (to the left) on the upper, or pattern control knob,  widens the spray pattern. Turning in reduces it to a cone shape  (as shown with dial set at 0). Once the pattern is set on the gun, the next step is to follow  the correct spraying technique for applying the coating to  the surface.  If the painter has never used a spray gun to apply a finish  coat of paint, and the aircraft has been completely prepared,  cleaned, primed, and ready for the topcoat, he or she may need  to pause for some practice. Reading a book or an instruction  manual is a good start as it provides the basic knowledge  about the movement of the spray gun across the surface.  Also, if available, the opportunity to observe an aircraft being  painted is well worth the time.  At this point in the project, the aircraft has already received  its primer coats. The difference between the primer and the  finish topcoat is that the primer is flat (no gloss) and the  finish coat has a glossy surface (some more than others,  depending on the paint). The flat finish of the primer is  obtained by paying attention to the basics of trigger control  distance from the surface and consistent speed of movement  of the spray gun across the surface. Spray Gun Operation Adjusting the Spray Pattern Applying the Finish Figure 8-11.  Adjustable spray pattern.  0 6 8 10 4 2 Dial at 10 Dial at 8 Dial at 6 Dial at 4 Dial at 2 Dial at 0 Dial Air valve Gun body Locking bolt Spreader adjustment valve Fluid needle valve 8-11