Page No 4
Terms to Describe a Poor Paint Finish
1) Orange Peel
This is a finishing effect so called because it resembles the
texture and appearance of orange
peel.
Common causes - unsuitable paint thinners
- atomising pressure too high or low
- material viscosity too high
- material not thoroughly mixed
- material too cold
- conditions in the finishing room (i.e.
draughts, temperature, humidity)
- poorly prepared surface or
contaminated surface
- holding gun too close or far from
surface
- applying too thin a coat
2) Runs and Sags
Common causes - too much material applied
- material being too thin
- tilting gun at an angle causing spray
pattern to strike surface unevenly
3) Streaky Finish
Common causes - tilting the gun
- split spray pattern
- incorrect overlapping of spray strokes
- spraying with heavy top or bottom
spray pattern
4) Mist or Fog
Common causes - over atomisation due to too high an
atomising air pressure
- improper use of the gun arcing or
swinging too far from surface, not
releasing trigger at the end of the
spray stroke
5) Starving
Common cause
- insufficient air or material feeding the
gun
the design and manufacture of spray guns that failure to
achieve a good finish is more likely to be attributed to poor
operator technique than equipment fault.
6) It should also be noted that a spray gun may have a
choice of nozzle and air cap combinations called
“settings”. Each setting will control the air consumption
and type and amount of paint that can be applied in a
given time. The incorrect choice of setting to paint type
can give poor results. Seek advice from the manufacturer.
which the operator of the gun should study closely,
practise and follow.
Firstly, the gun at all times should be held in a comfortable
position and at a distance of about 7 to 9 inches from the
surface to be coated and, most importantly, at right
angles to the surface. The working strokes should be
made in straight uniform passes backwards and forwards
across the surface in such a manner that the spray pattern
overlaps the previous by about 50%.
Secondly, the speed of the stroke must be constant to
maintain uniform thickness of coat and each stroke
should be made with a relaxed arm and flexible wrist so
that the gun is kept at right angles to the surface
throughout the whole operation.
Thirdly, trigger control should always be by a smooth
even pressure of the finger and never pulled abruptly or
released with a jerk. A little time spent in practising fine
pressure control of the trigger as distinct from using the
material needle adjusting screw, is amply rewarded
when complex shaped articles are to be coated. Correct
triggering of the spray gun is an essential part of the stroke
technique. To prevent paint build up at the beginning and
end of each stroke, the movement of the gun should
commence before the trigger is pulled and the trigger
released again before movement ceases.
Common Errors in Handling
1) Holding the Gun at the Wrong Distance
When the gun is held too close to the surface, the paint is
concentrated into a smaller area and unless very rapid
strokes are made, the surface will be overloaded
inevitably resulting in “sags” and “runs”. When held too far
from the surface, a coarse dry coating that has an
“orange peel” effect results.
2) Varying the Speed of the Stroke
A smooth uniform finish depends largely upon the even
stroke of an operator in relation to the volume of paint flow
and this should be regulated to conform to the operator’s
natural speed of movement.
3) Tilting the Gun
If the gun is tilted out of the vertical position this will result
in an uneven coat. It may also affect the paint supply if the
level of paint in a gravity or syphon cup is low.
4) Swinging or Arcing the Gun
This is perhaps the most common fault in handling a spray
gun and is caused by keeping the arm and wrist rigid
resulting in the gun swinging in an arc from the shoulder,
consequently not pointing at the surface at right angles
throughout the whole stroke, resulting in uneven paint
deposit.
5) Incorrect Overlapping
If the spraying strokes are too far apart and do not overlap
by about 50%, a streaky finish will result whilst too much
overlap can easily overload the surface. The perfect finish
will be only obtained if the many factors involved in spray
painting receive the necessary care and attention. It can
be taken for granted with the high degree of perfection in