Spray Finishing Basics

© Copyright by Thomas Craven November 2008 ©

All spray systems attempt to achieve one thing, that is atomization. Atomization from the dictionary is to, “reduce to minute particles or to a fine spray”. Whether you chose conventional, HVLP, or airless spray equipment, they all achieve the same thing; that is atomization. A satisfactory finish is produced when the spray material is atomized to the point that the material will flow onto the surface and become a uniform and even coating.

Viscosity and pressurization are the two main factors that effect proper atomization.

I looked up the definition of viscosity in the dictionary and found it to be just as hard to understand as the name itself. So I will tell you my definition of viscosity as it relates to spray finishing. Are you ready for it; it’s how thin the material is! Correct atomization occurs when the material being sprayed is just viscous or thin enough that the pressurization provided by the spray equipment in use can break up or vaporize the particles to produce a satisfactory coating. You only thin the material down enough so that your spray equipment will atomize the material while producing a coating that flows out onto the surface. Thinning the material too much will leave a coating that is spotty and runs down vertical surfaces. A coating too thick that is drying to fast will give you dry, rough areas and orange peel.

Now that we have our material correctly reduced; how do we then atomize it? By pressurization. It is the air under pressure coming out of the front of an air gun that breaks up the material. Or in the case of airless spray equipment, it is the pressurization of the material itself in the spray line that is then forced out the front of the gun through a very small orifice or spray tip that causes atomization at the front of the gun.

Spray Equipment

Conventional – High-Pressure, Low-Volume air spraying is out dated technology. Rarely in use inCalifornia any more. Produces a lot of over spray and hence wasted material. Material is presented at the front of the gun where air ports in the air cap at the front of the gun introduce a low volume but high pressure, (40 – 60 psi.), stream of air into the material to break it up and create a variable fan pattern.

HVLP – High-Volume, Low-Pressure, is the standard equipment in use today inCalifornia. It atomizes the material in the same manner as the conventional gun, by introducing a stream of air into the material that is presented to the front of the gun. However HVLP guns use a larger volume of air at a lower pressure typically 10-20 psi. Reversing the relationship between pressure and volume. This system decreases voluminous over spray and saves materials.

Compressors and Filters. Both of these spray systems need compressed air to operate. This air should be filtered as the action of compressing the air heats it up. While in the tank and hose, this air cools down. The moisture in the air then condenses in the tank and lines. In addition compressors will inject oil and other contaminants into the air supply. An oil and water separator / filter mounted close to the end of the airline is recommended. The compressor should be installed outside the building whenever possible. Remember it will be drawing air from the surrounding area you install it in and blowing it on your work.

Spray cups / guns and spray pots.

A siphon cup gun has a one-quart cup attached to the bottom of the gun. When you pull the trigger on the gun, air is released out of the air ports in the cap at the front of the gun. This air stream siphons / pulls material out of the cup. The vent hole at the top of the cup must remain free and clear to allow air to enter the cup so that the siphoning action can take place.

A gravity feed cup gun is a new innovation in cup / gun design. The cup is mounted above the gun so that the force of gravity is used to bring the material down and present it at the front of the gun. You must keep the vent hole at the top of the cup free and clear to allow the material to drain into the gun. It is superior to the siphon feed cup gun design. Both siphon and gravity feed cup guns are best used for small amounts of material. Sample work, shading and toning procedures, etc.

A spray pot is a two-gallon or sometimes five-gallon pot or bucket that has a pressure regulator attached to the top of it. The regulator allows air to free flow through to the gun to atomize the material, but it also pressurizes the material in the pot. Instead of passively siphoning or allowing gravity to feed the material to the front of the gun, the spray pot will deliver material under pressure to the front of the gun for atomization to take place. This will allow greater production as less air is needed to atomize more material. In addition greater capacity is provided by the two or five gallon quantity of material in the pot.

Airless spray machines use a hydraulic pump to pressurize the material that is sucked up in a pick up tube from a five-gallon bucket. The material travels in a ½” diameter spray line that is available in fifty foot lengths. The material is forced out the front of the gun through a very small orifice or spray tip. This spray tip atomizes the material and shapes it into a spray fan of preset widths 3 to 21 inches wide.

Air assisted airless is a hybrid system that combines the low pressure high volume air only atomization of hvlp systems with the high volume delivery capability of airless spray equipment. The material is atomized first by a single orifice spray tip under pressure. Further atomization occurs via an air cap on the end of the gun that introduces air into the material stream. The advantages are less over spray than conventional or hvlp systems with greater material flow and transfer efficiencies like pure airless systems.

The big advantages to airless spraying are, 1. Dispensing with the need for compressed air to atomize the material. You get straight material coming out of the gun without the cold, damp, dirty air that is delivered by most compressors. 2. A uniform spray pattern delivering a consistent amount of material all the time. This spray pattern does not require constant adjustment like the conventional and HVLP guns do. 3. It is a closed airtight system so that you may leave material in the spray rig and bucket for extended periods of time. 4. Only one material line too the gun allowing for more mobility. Excellent choice for high production shops.

Spray Techniques

Spray finishing skills take a lot of raw gun time to be proficient at. You can speed this process up with proper training and coaching. A very simple concept I express to my spray man trainees is; Spray until the surface just gets wet, no more and no less. If you spray too dry, the material won’t flow and will be rough. If you spray too wet, it will run and fall down vertical surfaces or pool and puddle. Your goal is to get the material to flow and become one even uniform coating with no orange peel, dry spots, runs etc.

Always start from the up wind end of the work and spray away from you towards the face of the booth. You don’t want to spray into the airflow of the spray booth as this will leave over spray on previously finished parts and the spray man too. Shoot all edges first and try to shoot your best surfaces last.

Cabinet boxes should be shot from top down, working from inside out and horizontal surfaces last. Start and stop your spray movement before and then after you get to the corners. This will allow your over spray to fill in the corners. Do not spray all the way into the corners of a box.  This usually results in to much material deposited in the corners and the ensuing dreaded runs.

Lapping the material. The general rule of thumb is to overlap your spray pattern by 50 %. This is not set in stone. It can vary depending on the speed you are moving your spray hand. Just remember the basic principal. Spray until the surface just gets wet, no more and no less. Be sure to overlap the leading and trailing edge of a panel to apply a uniform coating to the edge.

Six to eight inches away? Again this is a rule of thumb that is not written in stone. Hold the gun the distance from the surface that produces a coating that is just wet.

Watch the material coming out the front of the gun. Don’t watch the gun or the surface you are spraying. Keep your eye focused on the material flowing out the front of the gun. This technique gives you a better perspective on how much material is being deposited on the surface.

© Copyright by Thomas Craven November 2008 ©

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Tags: How to Spray Paint, Spray Painting, Spray Painting Technique, Spray Painting Tips