The air lines are clogged.
The air line is of an improper diameter.
An inadequate air supply. This refers to the volume of air that is being delivered by the compressor, and
not necessarily the pressure at which the air is delivered.
The air intake is clogged.
5-6. CARE OF SPRAY EQUIPMENT
General. A spray gun is an instrument that has been designed and machined to close tolerances. Handle it with
care so that the balance between the functional parts is not destroyed. Spray guns and related equipment require
cleaning immediately after use. Paint that has hardened in a gun or hose is extremely difficult to remove, and usually
causes a malfunction of the equipment. Be sure that the solvent used to clean the equipment is one in which the finishing
material is soluble. Be sure to read the instructions that come with the pressure can regarding preservation of the nozzle.
Suction Cup Type. Remove the cup, keeping the fluid stem inside the cup or container, as shown in figure 5-10,
then hold a cloth over the air cap and pull the trigger. This directs air into the fluid passageways, and blows any paint that
may be in the gun back into the container. After cleaning out the cup, clean the gun by spraying a small amount of clean
naphtha or thinner through it. Extreme care should be exercised in the removal of the fluid top so as not to split the tip or
otherwise injure it or the fluid needle. When loosening the fluid tip, the trigger of the gun should be compressed so that
the needle is not in contact with the tip; this eliminates the possibility of splitting the tip due to friction or sticking that is
caused by dried paint. The entire spray gun should never be immersed in naphtha or thinner, as this removes lubricants
and dries out the packing. Under no circumstances should the air or fluid ports of a gun or nozzle be reamed with any
substance harder than soft wood, as a deformation of the spray pattern may result, and the spray gun may be rendered
useless. To prevent wear, the working parts of guns should be kept lubricated with light machine oil. This requirement is
especially true of the needle. The needle packing should also be kept pliable with an occasional drop of light lubricating
Pressure Feed Type. Shut off the air supply to the pressure tank (see figure 5-11), release pressure in the tank,
and blow back fluid as in the suction cup equipment (paragraph 5-6b above). Empty and clean the pressure tank. Place
a container of clean naphtha or thinner inside the pressure tank and install the lid, making sure that the fluid delivery tubes
(see figure 5-11) are immersed in the container of naphtha or thinner. Apply pressure and operate the spray gun to clean
the hose and spray equipment. Disconnect the fluid hose from the gun and pressure tank and allow the hose to dry
thoroughly before reconnecting. Clean the air cap and fluid tip as in the suction-cup equipment (paragraph 5-6b above).
5-7. TOUCHUP PAINTING
General. When equipment or material has spots from which the protecting paint has disappeared, and the rest of
the paint surface is in a satisfactory condition, it is often advantageous to do a touchup, rather than a complete, painting
job. The bare spots may have been caused by natural wear, abrasion, mechanical injury, or by rust or corrosion of the
surface under the original paint. In such cases, it is necessary to clean the material beneath the spots and repaint using a
method as near as possible to that used on the original paint job.
Cleaning. The spots to be painted must be thoroughly cleaned so that no decay, dirt, rust, corrosion, etc.,
remains. The remaining paint should also be worked down to a feather-edge if it is desirable to hide the lap.
Painting. While touchup painting may be done by the brush method, spraying is superior because the edges of
the new paint can be feathered out to blend with the old surface. If the old and new colors match, the areas of the new
paint will not be noticeable. In touchup work, use fillers, primers, and paints that are compatible with the existing
undercoats, finish coats, etc. The general instructions for spray painting given in paragraphs 5-1 through 5-5 are also
applicable to touchup work.