inspected. A thin, even coating with no breaks or non-wetted area is necessary. Excessive penetrant is not desirable as
it tends to run or drain off the area and complicates removal. Holding the can motionless or moving it too slowly while
spraying will result in an excessive layer of penetrant. Short distances between the can nozzle and the part reduce the
size of the spray pattern, and produce a thick layer of penetrant in a small area. Long distances increase the size of the
spray pattern, and reduce the penetrant layer thickness. There is also an increase in overspray and the possibility of
Brush or Swab Application.
Care must be taken to avoid spilling the penetrant while on or in an aircraft.
Penetrant can be applied to large parts by brushing, wiping, or even pouring from a container. The brush or swab
method is most frequently used to coat a small area of a large structure. Brushing or swabbing provides control over
the placement of penetrant on the desired area; improves the ability to regulate the quantity or thickness of the
penetrant layer; and eliminates overspray. Any sort of brush, swab, rag, or even sponge can be used (synthetic sponges
may dissolve in penetrant). The size of the brush can vary from large paint brushes down to small acid or artist
brushes, depending on the size of the area to be covered. Any type of clean container can be used to hold the penetrant.
Penetrants may be applied over a range of ambient temperatures. However, certain limits must not be exceeded as the
inspection process may be degraded. The operating range for conventional penetrants is 40°F (4°C) to 120°F (49°C).
There are special penetrants formulated for hot applications exceeding these limits. They are discussed in Section 2.8,
Special Purpose Materials, paragraph 2.8.4.
Low Temperature Limitations.
Penetrant inspection SHALL NOT be performed when the test part temperature is less than 40°F (4°C). There are
several reasons for this restriction:
a. At 32°F (0°C) or less, any moisture, even from the inspectors breath, will form ice crystals on the part,
which will interfere with the penetration process.
b. The propellant pressure in aerosol containers is affected by temperature. The gas pressure decreases
with lower temperatures. When the temperature drops below 60°F (15.6°C), the reduced pressure can
result in an erratic spray pattern.
c. The evaporation rate of solvent cleaners and nonaqueous developers is reduced at lower temperatures.
Figure 2-11 shows the evaporation or drying time for two types of nonaqueous developers at various
temperatures. The graph shows a ten-fold increase in drying time between the temperatures of 60°F
(15.6°C) and 0°F (-18°C).
d. The viscosity of the penetrant increases as the temperature decreases. When temperatures are between
40°F (4°C) and 60°F (15.6°C), the penetration dwell time must be increased due to the increased
viscosity. Additional information on temperature-viscosity is provided in paragraph 18.104.22.168.2.5. The
increase in solvent cleaner evaporation time; penetrant dwell time; and developer drying time required
at temperatures lower than 40°F (4°C), makes the total inspection time far too long to be practical.