The Spray Gun
The best quality spray guns obviously produce the best results. Devilbiss make some of the best guns on the market and some cost many hundreds of pounds, but you can pick a new one up on Amazon for around sixty pounds. Even if these are the cheapest guns Devilbiss make, I would be surprised if they could not produce excellent results. Although the cost of spray guns and auto paint may seem high, the cost of a professional respray will amount to many times more. The spray gun shown on this page is the one I used on my own Morgan. Although there are different types available, the principles of setting up and maintaining spray equipment, will be similar to those described below.
The spray gun shown in Figure 2 is a siphon type. When the trigger is squeezed, the fluid needle retracts, allowing air to flow through the top of the gun. An area of low pressure is created, which lifts paint from the cup. The Air travels along its own passage whilst the paint flows along the passage containing the fluid needle. As the tapered fluid needle retracts, air passes through holes in the the fluid nozzle body and is forced through the holes in the air cap. Paint flowing from the nozzle tip is atomised by the air flowing from the holes in the air cap. Air flowing from the horns of the air cap force the atomised paint into a fan shape. This type of gun uses relatively high air pressure and creates a fair amount of over spray. Typical air pressure used with this equipment ranges from 40 to 45 psi.
Although siphon guns are still used, most paint finishing today, is achieved using HVLP (High Volume Low Pressure) gravity feed guns. Siphon guns are easy to use, but they are not as efficient as HVLP gravity fed guns. The advantages of HVLP equipment are, less air pressure, less overspray, economical on paint, better finish from the gun and easier to clean. The paint cup is on top of the gun in a gravity feed, which means it does not have to be removed when cleaning it. However setting the gun up is similar to a siphon gun, which is the type referred to in the following text. This is the only equipment I have experience of.
Setting the Spray Gun Up
Before putting any paint in the gun, switch the air outlet tap off and connect the gun to the air line. Undo the material valve control knob until the trigger can be pulled fully back. Open the sideport control knob a few turns. Switch the compressor outlet tap on and pull the spray gun trigger fully back. Adjust the water separator regulator to the recommended outlet pressure for the paint you wish to use. For spraying cellulose with a siphon gun, this should be 40 - 45 psi. DO NOT set the regulator pressure above 50 psi. Release the trigger and check the gun has no air leaks.
The viscosity of the spray paint must be correct to achieve a good spray job. Check the manufacturers data sheet and mix the cellulose paint with anti bloom thinners in a separate container. If no sheet is available, cellulose and thinners is usually mixed 50:50. "2K" acrylic is mixed, 2 part colour, 1 part hardener and 10% thinner. Fill the spray gun paint cup to a maximum of 75% capacity. Make sure the locking ring is properly tightened when fixing the paint cup to the spray gun.
When buying a spray gun, check it is suitable for the material you intend to use. If you look at the horns on the air cap, you should see a number which is the "nozzle" code. This nozzle code on my old Binks Bullows 222 sray gun air cap is "66S2". The data sheet that came with the gun informed me that this nozzle would consume 6 c.f.m. and was suitable for applying a range of paints, including cellulose top coat and primer fillers. This enabled me to purchase the correct compressor, which was capable of continuously supplying air for spraying the products that I intended to use.
From a distance of 20 to 25 centimetres, spray paint onto test paper taped to the garage/spray booth wall. Adjust the sideport control knob clockwise until a circular spray pattern is achieved as shown in Figure 3B
Slowly adjust the sideport control knob anti-clockwise until a 25cm long oval spray pattern is achieved as shown in Figure 3A. It may be necessary to play with both controls until the correct pattern is achieved.
A new, decent quality spray gun should be easy to set up and trouble free. However, if it is not carefully looked after, it will soon perform poorly. Paint must be mixed with thinners to achieve the correct viscosity. Always filter paint before filling the spray gun and make sure the surface to be painted is free from dust, oil and products containing silicon.
ALWAYS CLEAN THE GUN IMMEDIATELY AFTER USE. If the spray gun is not meticulously cleaned after use, it will not produce good results the next time it is used. I have never used "2K" paint, but apparently when it dries, it is extremely difficult to remove. Cellulose dries quickly but is easier to clean.
Cleaning the Spray Gun
After a spraying session, any paint mixture left in the paint cup should be emptied and replaced by thinners, which should be sprayed through the gun immediately. The cup should then be removed from the spray gun and any thinners left, can be used to help clean it. When it has been thoroughly cleaned, put fresh thinners in the cup and flush it through the gun until the spray runs clear. Paint on the outside of the gun can be wiped down and removed with a rag soaked in thinners. DO NOT immerse the complete spray gun, otherwise lubricants and packing in it will be affected. NEVER use wire or metal tools to clear blocked holes in the air cap. The holes are accurately machined so that the paint can be perfectly atomised. Undo the locking ring and remove the air cap. Immerse it in a container of fresh thinners and use a bristle brush to thoroughly clean it. Pay particular attention to the face and side port holes. Make sure there is no dried paint restricting them. Removing the air cap will reveal the fluid nozzle, which houses the fluid needle. Check that there is no dried paint on the needle and retract it by squeezing the trigger. Carefully examine the orifice in which the needle is seated and make sure there is no dried paint present in or around it. Finally blow dry air through the spray gun and store it in a clean dry place.
Identifying and Correcting spray faults
Nowadays HVLP (High Volume Low Pressure) gravity fed guns are the most popular to use, because they produce less over-spray and therefore consume less paint. Most of the information here is based on using a siphon type spray gun as shown in Figure 2. I assume details about paint viscosity and spraying pressure will be different for HVLP gravity fed guns. However the following principles on setting the gun up should be the same. If you use an HVLP gun, relevant data sheets should be observed to select the correct operating conditions.
Time spent test spraying a sheet of card or paper, taped on the workshop wall, will enable you to spot faults before spoiling the paint job you are aiming to accomplish. Assuming the gun is in good condition and the paint is correctly mixed, the first thing to do, is to produce the correct pattern on the test paper. The aim is to produce a neat oval spray pattern as illustrated in Figure 4 (A) It should consist of an even depth of paint over most of its area.
If you are using a siphon gun, the air regulator pressure should be set at 40 - 45 psi for spraying cellulose paint . The gun must be held square to the paper at a distance of approximately 200 - 250mm (The span of your splayed open hand from thumb tip to small finger tip). The air cap horns should be horizontal and the length of the pattern should also be between 200 and 250mm, unless you are painting a small area. If this is the case reduce the air flow by turning the side port control clockwise or the material valve anti clockwise.
If the pattern appears round, the air pressure is too low. Turn the side port control anticlockwise
The pattern shown in Figure 4 (C) is caused by a blocked side port hole in one of the horns on the air cap. Dry paint in the left hand port will cause the pattern to be distorted to the left side and a blocked right hand side port will cause the pattern to be distorted towards the right. Immersing the air cap in thinners, then blowing it through should solve the problem.
Faulty spray pattern Figure 4D results from the air pressure being set too high, or inadequate material flow. Turn the side port control knob clockwise and the material valve control knob anticlockwise. Too much thinners in the paint is another possible cause.
Figure 4E is usually caused by dried paint around the fluid nozzle tip, but it may also be caused by a loose air cap. Remove the air cap and clean it with thinners. Clean the air tip, then screw the air cap on tightly.
Having adjusted the gun to obtain the correct spray pattern, it is important to use the correct technique when spraying the car. The spray gun paint tip should be a hand span in width (approx. 200mm) away from the panel being painted. As it is swept back and forth, it must remain equidistant and parallel to the paint surface, as illustrated in Figure 5A. Unfortunately novices tend to swing the spray gun in an arc, causing the paint coat to be heavier in the middle.
Sweeping the gun through an arc is caused by keeping the wrist locked. Figure 5B shows a centre line drawn through the hand and the forearm. The line is straight in all positions, because the wrist is locked and the elbow is used as the pivot point when the gun is swept from side to side. Figure 5A shows the centre line is bent on each end of the swing, but straight in the middle. This is because the wrist is not locked and is used as a pivot to keep the spray gun at 90 degrees and equidistant from the paint surface.
As the gun is swept from side to side, each coat of paint should overlap the last by 50 percent. This helps achieving a good even thickness. Cellulose will usually flash of in 15 minutes if applied at an ambient temperature of 20 degrees centigrade. The next coat can then be applied. Try to apply it at right angles to the first, as this will also help in achieving an even thickness. If you are spraying a vertical surface do not use the gun sideways. Rotate the horns on the air cap, by rotating it through 90 degrees.
An uneven paint surface often leads to paint sagging on vertical and curved surfaces. Paint sags should be avoided at all cost. However, if they occur, do not rub them down as soon as the paint flashes off. Leave it to harden for 3 to 4 days, then use 1200 grit wet and dry paper on a rubber sanding block to flat it back. Auto paint is expensive and if you have not used a spray gun before, it may seem wasteful to use a cup full of paint trying to perfect the technique, but it will pay dividends later on. It will help produce a good finish from the gun and minimise time wasted rubbing down and correcting paint faults.