If you look up the word “Practice” in a thesaurus, you will find that it can have many meanings, such as Preparation, Training, Habit, Routine, Procedure, to Perform, to Apply, to Follow, to Observe, and TO DO.

“Good Practice” can therefore mean that we must prepare well, have adequate training, get rid of bad habits, follow established routines and procedures, perform to the best of our abilities, follow specifications and observe rules and regulations, simply we DO OUR BEST.

On a practical level, good practice within surface treatment will include:

Planning & organisation

• Make sure all necessary documentation is up-to-date and available, including copies of specification, relevant standards and data sheets

Accessibility

• Good access is as important to achieving satisfactory coating application as is the quality of the equipment!
• If a paint applicator cannot reach the areas to be coated in a comfortable and secure way, the quality of the application is bound to suffer

Personal Protection Equipment (PPE)

• Use correct and adequate protection while using paint

Proper cleaning of tools after or during application

• Cleanliness and good housekeeping before, during and after application are some of the most important factors toward achieving a good result
• Nothing can stop two-pack paints from curing once they have been mixed, while single pack paints will start to dry when exposed to open air. Application equipment which is not cleaned immediately after use will quickly become useless and completely unsuitable for further application of paint

Application methods and equipment

There are a number of application methods and equipment available for paint application, such as Painter’s glove (for small pipes & handrails), Electrostatic spray, Pressure-feed brush & roller, Dipping (production line), “Curtain” application (high volume sheet or coil operations), Counter-roller (coil coating), Sponge (decorative) and Under water application.

Structures requiring heavy duty corrosion protection can be of complex shapes and manufactured from a variety of materials such as carbon steel, aluminium, plastics etc. and can have very large surface areas that need protecting. Some common methods and equipment are:

• Brush application
• Roller application
• Air spray application
• Airless spray application

We will focus on these methods and related equipment in this course. They include both hand tools as well as power-driven application methods.

Hand-tools for application of paint

• Stick for stirring paint: Only to be used for small tins, power-mixer must be used for stirring larger tins of protective coatings
• Paint brushes (various sizes and shapes)
• Paint rollers (various sizes and shapes)
• Tape for masking

Application by paint brush

Benefits

• Good wetting of the substrate
• Forces the paint into the surface
• Better than a roller on the first coat
• Good on areas with poor accessibility

Limitations

• Gives low film thickness, many coats required
• Creates an uneven film
• Application speed is slow

Good quality paint brushes must be cleaned properly to maintain their properties

Application by roller

Benefits

• Application speed is faster than with a paint brush
• Good on areas with poor accessibility

Limitations

• Poor wetting of the substrate
• Never use for the first coat as the roller may trap air and cause pinholes in the paint film
• Gives low film thickness, multiple coats required

Roller application will not force the paint into the profile of the substrate, but rather deposit it over the peaks. Satisfactory result for industrial/marine painting is very dependent on the quality of the surface preparation.

The principle of spray application equipment is that paint is forced through a hose to a spray gun, where it is released through a nozzle with a small opening. Upon leaving the nozzle the paint must be broken down into small droplets and pushed forward at speed. This is achieved in a few different ways depending on the type of spray application equipment.  

Below we will present the following:

1. Air Spray
2. HVLP: High Volume Low Pressure
3. Airless Spray 
4. Air assist airless spray
5. Plural-component airless spray pump
6. Nomogram

Air spray is often also referred to as “conventional spray”. When the paint exits the air spray nozzle, it immediately enters a crossfire of pressurized air jets which breaks the paint into small droplets and push them forwards. Air spray applied paints thus form a mist of fine droplets.

There are three main principles for making the paint move through and out of the air spray nozzle:

1. Gravity feed: The paint is held in a container which is mounted on top of the spray gun. Gravity will force the paint into the spray gun and nozzle
2. Siphon feed: The paint is held in a container which is attached to the underside of the spray gun. A pipe leads from the paint in the container to the inside of the gun. When compressed air passes through the gun, the paint is siphoned (drawn) from the container to the spray nozzle
3. Pressure pot: The paint is held in a separate closed container which is connected to the spray gun via a hose. The pressure in the closed container is increased by letting in compressed air, causing the paint to be forced through the hose to the spray gun

Air spray application is only suitable for thin paints (low viscosity). Such paints may suffer from pigments settling in the lower part of the containers, particularly of the paint contains heavy metallic pigments (e.g. zinc silicate paint). Many pressure pots have a built-in stirrer, driven by a separate air motor, to prevent such pigment settling.

Benefits

• Higher productivity than hand tools
• Admirable finish-smooth and glossy
• Lower pressure-safety
• Even film thickness
• Simpler equipment

Limitations

• May have dry spray (overspray)
• Limited film thickness
• Only for low viscosity paint
• Relies on compressed air

Conventional air spray guns operate at a basic, high compressed air pressure. They connect to an air compressor and the finish is "blasted" onto the substrate using this high pressure. Because this high-pressure finely atomizes the spray (produces very fine particles), this gives a very nice smooth application, typically used in the automotive industry. Despite this excellent end result, conventional spray guns are only about 30% effective, meaning as little as 30% of the paint actually ends up where it‘s supposed to go.  The rest is overspray, bouncing off the substrate or simply floating in the air. The high-volume low-pressure spray guns increase transfer efficiency, which means that much more of the paint is actually transferred to the substrate instead of being wasted. With the HVLP system, air is delivered at much higher volume, but with a much lower pressure, so that more paint ends up on the substrate instead of in the air.

There are two different types of high-volume low-pressure spray gun systems on the market. In the first type, the spray gun still runs off an air compressor just as a conventional spray gun does, but it uses much lower pressure. The second type of high-volume low-pressure spray gun is the most efficient. This spray gun uses an air turbine instead of air compressor. This delivers an extremely high volume of air at very low pressure.

A special pump will suck the paint out of the tin and deliver it at high pressure through a spray hose to the spray gun. When the spray gun trigger is pulled, the paint will be released at high pressure through the nozzle on the gun. Inside the hose and the gun the paint is kept at this high hydraulic pressure, but when it exits the nozzle there are no boundaries to confine and maintain the pressure. The sudden drop in pressure will rip the paint apart, resulting in a large number of small paint droplets being flung into the air. This is what is called “atomization” among spray painters.

The nozzle in the airless spray gun can be removed and replaced. Many different nozzles are available, each with their own design and properties.

• They are all made with an elliptical opening so the paint will leave the gun in a flat “V”-shape formation, called the spray fan. The fan may be very broad or quite narrow, which is expressed as the “angle” of the nozzle. For large flat areas you may use a large (wide) fan, while areas with a lot of details may be best painted with a narrow fan. The fan is given as the number of degrees the V-shape fan is (e.g. 40^o, 60^o, 80^o, etc.)
• The amount of paint coming out of the nozzle at a given pressure will depend on the opening size, called the “orifice”. A high-build, high-solid paint may work well with a reasonably large orifice, while a glossy topcoat would give the nicest result after being applied by a smaller orifice. The size of the nozzle may be given in the metric system (parts of a millimetre, e.g. 79 mm) or the imperial system (parts of an inch, e.g. 0.031”).
• The recommended nozzle to be used for airless spray application of a paint can usually be found in its TDS. Remember that nozzles get worn, particularly when applying paints containing hard pigments, so they must be replaced regularly!
• Using the wrong nozzle may result in poor atomisation (spitting) or blockage of the nozzle. It is now common to mount the nozzle in a cylinder which can be rotated 180^o. If a blockage occurs, the cylinder can be rotated and the impurity blown out of the nozzle with a quick squirt and rotated back to the correct position again.

The pump that sucks the paint out of the tin and creates the high pressure in the paint inside the spray hose and spray gun is normally a double-action piston pump. Double-action means it deliver pressure on both the up-stroke and down-stroke of the piston. The pressure is maintained by means of ball-valves seated at the bottom of the pump house (cylinder) and inside the piston. This pump can be driven by an electrical motor, but the most common types for heavy duty application are driven by an air motor. Compressed air is much safer to use near flammable solvents than an electrical motor which creates sparks (unless of an explosion-proof type). The air motor is constructed such that compressed air will push the piston both up and down, directed by a slide valve. The air motor piston and the pump piston are permanently connected by a common piston rod.

All airless spray pumps that are driven by an air motor will have a “ratio”. This ratio refers to the size of the piston in the air motor versus the size of the piston in the paint pump. If the ration is for example 48:1, it means that the surface area of the air piston is 48 times as big as the surface area of the paint piston. The consequence of this is that if the compressed air which drives the air motor is 5 kg/cm² (0.5 MPa), the pressure of the paint leaving the pump will be 48 times as large, i.e. 240 kg/cm² (24 MPa). In addition to the recommended size of the nozzle, paint manufacturers will also recommend what pressure the paint should have at the nozzle in order to obtain the most suitable atomization. It is worth to remember that the paint pressure will drop due to friction in the paint hose. This can have some influence on the result if the paint line consists of several lengths of hose connected together, and particularly so if the pump is situated at ground level and the spray painter is working high up in a scaffolding (pressure is lost while pushing the paint in the hose against gravity up the difference in height, particularly for heavy paints containing metallic pigments).

The hose connecting the spray gun and the spray pump must be of an approved type, including the couplings used to connect to the pump and the gun (and to any extension hose). It is important that there is a continuous conductive connection between the gun and the pump in order to avoid electrostatic build-up and possible sparks. The hose must therefore have a functioning earth lead built into its wall.

Application by airless spray

Benefits

• Very high productivity:
  • Applicator saves time: Up to 50 - 75% compared to other methods
  • With paint brush: Application takes four times longer
  • With roller: Application takes twice as long
• Can apply high viscosity paint
• Higher thickness in a single coat
• Even film thickness

Limitations

• Dry spray (overspray)
• Safety-high pressures
• Higher loss for smaller sized objects
• Complicated and expensive equipment
• Skilled applicator required

How to get the most out of your airless spray tip / nozzles

• Use the lowest amount of air pressure required to atomise the material and to provide a satisfactory spray fan
• Sieve all material
• Use filters (unless manufacturer advises to remove them)
• Remove the spray tip before cleaning spray gun
• Clean spray tip with brush
• The equipment must be cleaned after use
• Two component paints will continue to cure / harden in pumps, lines and gun and must be removed before they become solvent resistant

Both airless and air assist airless spray guns use high fluid pressure to atomize. Paint is forced through an elliptical shaped orifice. This high pressure allows the coating to be forced through the entire fluid tip, including the narrow edges of the elliptical shape. Such high pressure, while allowing complete atomization, will cause an increase in material flow, an increase in material bounce-back and increased fluid tip wear. The appearance of the applied film may be influenced by the high pressure as well, often resulting in a slightly “textured” finish (e.g. “orange peel”). One solution to avoid this is to lower the paint pressure. However, as the paint pressure goes lower, it will have difficulties exiting the sharp corners of the orifice. The result is what is commonly called tails (rat tails) or fingers in the spray pattern.

The solution to the “texture” and “tails” is to add an air cap to the spray gun. A properly designed air assist air cap will provide the necessary air in the proper direction to eliminate the tails in the spray pattern and reduce the texture in the finished film. These benefits all add up to the ultimate finishing tool for all applications where a superior finish is required together with high output, such as for example in the furniture industry. The amount of air assist will vary with the tip shape and size, paint pressure and paint viscosity. Exceeding the minimum amount of air required will increase bounce-back or overspray of the coating.

If the air pressure is kept to 10 psi or less as measured at the air cap, the gun may be classified as high volume low pressure (HVLP) in some environmental regulations.

Airless spray application of two-pack paints can raise a few challenges. Once the two components are mixed, the curing reaction will start immediately. For some paints, the mixture can be used for a few hours (pot life), but for other paints it may be only a matter of 30 minutes (or less in warm weather) before the mixture has cured so much that it cannot be applied any more. But it is not only a matter of having sufficient time available for application, it is also a matter of emptying the airless spray equipment and clean its interior before the mixture hardens and destroys the equipment. Any paint which has been mixed but not applied within its pot life will of course be a complete loss.

Plural component airless spray equipment is a device with two separate pumping systems to supply the base and curing agent. There are two types available: fixed mixing ratio and variable mixing ratio. The two components may be mixed as they leave their respective pumps or transported in separate spray hoses and only mixed just before they reach the spray gun (in-line mixer). They are particularly suitable for:

• High viscosity paint
• High volume solids paint
• Paint with short pot life

The equipment can be fitted with heating systems to lower the viscosity of thick, solvent-free coatings and for other coatings in cold weather.