Car Painting Tips: Air Problem in Spray Guns Painting

Correcting Spray Gun Air Pressure Problems

As long as you clean your spray gun and leave it in a place where it is unlikely to be injured by other objects being set atop it, and the like, you can expect many years of faithful service from this device, and will probably be using it long after many of your other current tools have worn out and been replaced. Some painters report using a single spray gun for twenty-five years or more – although, of course, these are well-engineered American models which use good material, and modern Chinese models, frequently made of pot metal, may be less enduring.

Nevertheless, spray guns are not perfect or infallible, and there will inevitably be moments when they fail you. Parts will be misadjusted or improperly lubricated, one of them will occasionally break and need to be repaired or replaced, and so on. Besides the air nozzle and air cap cleaning problems described in the previous article, there are some larger problems as well. These can be classified into two sets – air pressure problems and material (paint, primer, etc.) problems.

Like a worn-out lawnmower that continues to sputter after the safety handle has been released, air may continue to flow after you have released the trigger. This may happen for only a few seconds or, in more vigorous cases of this problem, may go on more or less indefinitely. The root of this difficulty is the air pressure needle – located directly forward of the air micrometer.

If the air flows even when the trigger is not being squeezed, then the air pressure needle is sticking in the “on” position. To correct this, loosen the air micrometer until it comes free and pull out the air needle. Clean this carefully with solvent, then lubricate it with special spray gun lubricant. Using any other kind of lubricant is an open invitation to creating fisheyes in your paint – small or large discs of discolored, distorted paint that truly ruin the appearance of a painted car.

A well cared for spray gun can last for many decades, even a lifetime – after all, it is not subjected to high stresses like a hammer, a saw, or any angle grinder, and spraying paint is involves little sharp contact with other surfaces and no pressure or strain.

More Air Problems with Spray Guns

If the air leaks out around the fittings – places where the different components of the spray gun fit together, such as the air hose attachment or the air cap – then the cause may be incorrect tightening; this is easily corrected by screwing the hose, air cap, etc. into place more firmly. Before screwing the fittings on more tightly, however, you should check to ensure that they are not improperly-threaded, with the fitting attached askew. This is all too easy to do, and trying to force the fitting on more tightly while it is cross-threaded can produce severe damage to your paint gun.

Alternatively, the threads may just be permanently loose, possibly because of wear over time or poor machining straight from the factory. In this case, the corrective measure is to wind a little Teflon tape around the threading, then screw the fitting into place over it – the tape will act as “chinking” to keep the air from escaping. This is a method often used in plumbing, and what is effective for water is also effective for air. However, be very careful when cleaning the fitting coupling to remove all scraps and traces of the Teflon tape, lest it return to haunt you by clogging an orifice in the air cap or even the nozzle itself.

If the paint cup is pressurized and air leaks from it, then the problem may be dirt or wear. The gasket sealing the cup should be examined – cleaned if dirty and replaced if simply worn. Generally speaking, air leaks are less numerous than material leaks, although some material problems are also caused by air system difficulties.

The hiss of escaping air is not a sound any painter wants to hear when they do not have their fingers on the trigger of their spray gun. That slight, sinister noise indicates that something is seriously amiss with the device, which must be rectified if the implement is to work properly. After all, strong, steady, predictable air pressure is the foundation of spray gun operation, so unpredictable variations in the air pressure – or loss of air pressure – can seriously harm performance and the quality of the finished paint.

Additional Materials Problems – Fluttering Spray and its Kin

If the spray comes out in a “fluttering” manner – difficult to describe, but unmistakable when seen – then a whole laundry list of causes could be at the root of this effect. The material might be too viscous for the nozzle you are using, and is thus being forced through in spurts rather in a steady stream – in this case, the “cure” is to either thin the material or use a larger nozzle. Alternatively, partial blocking of the vent port in the paint cup, where air is drawn in from the outside, might be responsible, and this should be cleaned out if any obstruction is visible.

Fluttering spray can result from low amounts of paint remaining in the cup, which is corrected, naturally, by refilling this reservoir. Loose needle packing can prompt a flutter in the spray, so the packing nut should be tightened and, if this fails to work, the packing itself replaced. An air leak can also be the cause of fluttering, and this should be checked for and corrected with either tightening or Teflon tape as described in the article on the subject.

If the paint, primer, or other material refuses to come out of the gun at all, or does so very feebly, then the prime suspect is a clogged nozzle. Remove the air cap and then the nozzle, and clean it thoroughly with solvent. Alternatively, check the fluid pickup tube inside the paint cup for clogging and clean it if necessary, and make certain that the air vent in the cup’s top is open.

A problem unique to gravity-feed spray guns mounting the paint cup above the nozzle is the paint bubbling inside the cup, which throws off its usual properties. The cause is a loose nozzle – remove the air cap to tighten the nozzle, then replace the air cap and resume shooting.

Some materials problems are even more serious than those which were just described; these hindrances can prevent the paint gun from working at all until they are addressed, or cause such huge flaws in the paint application that the spray gun might as well be non-functional.

Correcting Leaks and other Material-Related Problems

The most obvious material problem is if paint or primer leaks out around the nozzle area, and this can be quite harmful because of the possibility of blocking the orifices in the air cap or the wing ports, possibly irrevocably if they are not cleaned in time. One of the most common causes is that paint needle valve is sticking open slightly, allowing some paint to dribble out when it should not.

The needle passes through an area of packing, which may become compacted and grip the needle too hard. Remove the needle valve by screwing the material control knob looser until it comes free, and then clean the needle, lubricate it with purpose-made spray gun lubricant, and loosen the packing slightly.

A compression spring is also built into the paint needle valve, allowing it to spring back into place when the trigger is released. If this has become flabby and worn out over time, it may no longer push the tip of the needle fully into the nozzle tip, allowing paint to escape around the needle. The spring needs to be replaced in this case, and the needle tip and nozzle should also be checked for damage that causes them to fit together improperly. Replacement is the only answer in either case.

If the material leaks out around the packing – which is roughly halfway between the trigger and the nozzle, on the underside of the gun – then the packing nut, or gland nut as it is sometimes called, needs to be tightened to hold the packing in place more firmly. If this does not work to stop the leak, then the packing nut should be removed and the packing itself replaced. The packing is needed to keep the paint from flowing back along the paint needle valve’s channel and dripping out near the trigger.

The other “family” of problems with a spray gun involves the materials being sprayed – primer, paint, clear coat, and so on – rather than the air supply itself. There are more of these problems than air system faults that a painter may need to contend with, and solving them is generally more complex because of the multiple possible causes for each specific difficulty.


References:
How To Spray Paint Your Car - Auto Painting & Bodywork Repair
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