Water base coating technologies have dramatically improved in the last decade. However, they are very diverse in formulation and may not spray the same from one material to another. Unfortunately, water base coatings require a diverse range of gun technologies and setups for each specific coating chemistry and application behavior. Here are some key coating characteristics to identify before selecting the application equipment.
1. •Viscosity of the coating
2. •Sheer sensitivity of the coatings
3. •Volume of solids in the material
4. •The co-solvents use in the formulation of the coating
For this article, we will discuss water base clear coats and paints that are non-UV cured.
Air-assisted airless equipment
In the best case scenarios, air-assisted airless medium pressure systems are the most ideal. They are best for transfer efficiency, break-up of high viscosity coatings, and producing medium-to-small particle size. They are also capable of high flow rates for increased productivity. For these reasons, we suggest you first try this type of system with your water base coatings.
Use a minimum pump size with a 15:1 ratio to handle the high viscosity characteristics of water base coatings. This means a 15:1 ratio pump will deliver 15 pounds of fluid pressure for every pound of compressed air applied to the pump. The fluid section of the pump should be sized larger than necessary for solvent based coatings. The minimum size of the fluid size will be determined by the manufacturer’s pump design and the characteristics of the coating you are using. Check with your equipment manufacturer for fluid selection recommendations. These larger pump systems should also be fitted with a suction hose assembly that will accommodate the heavier type of water base coatings without causing the pump to starve for material and causing the system to cavitate.
The air-assisted airless gun tip should be designed for water base coatings. A water base tip design will typically have a pre-orifice to assist in the break-up of the material. Tip sizes generally range from 0.0011 to 0.0015 inch orifice opening. We recommend using some trial-and-error to test which tip size configuration works best.
Typically, fluid pressures should be set between 500 and 700 hydraulic lbs. The atomization air pressure should be set between 15-25 lbs. Again, fluid and atomization air pressures will need to be adjusted on a trial-and-error basis until the best finish quality and application process is determined. As the coating technology improves, these parameters will be more standardized from one material technology to another. As of now, these parameters are very diverse from one material to another and cannot be determined without extensive testing.
Low pressure systems with air-assisted airless guns
Some water base finishes will not spray successfully through an air-assisted airless system using any type of setup or configuration without experiencing micro foam or other undesirable results in the finishing process. We then recommend testing a low pressure delivery system such as a diaphragm pump or a pressure pot fitted with an air-assisted airless gun. Gun tips should still be designed for water base coating. However, the tip orifice size opening should start at 0.0015 inch and possibly go up to as large as a 0.0016 to 0.0017 inch tip. Heavy viscosity coatings will require a larger tip size and formulations that are of lower viscosity. To operate this system successfully, fluid pressure on the pump or pressure pot will range from 60-90 psi. Make sure your delivery system will accommodate this pressure range. Atomization air will range from 18-28 psi. The atomization pressure should be set at the lowest value possible to successfully eliminate the “tails” in the fan pattern and produce a particle size to a level that will maintain an acceptable quality of finish.
Low pressure system with HVLP gun
If this system still does not eliminate micro foam or does not break up the coating sufficiently, the next option is to use the same low pressure delivery system and fit it with an HVLP gun. Gun tip size will range from 1.4 mm to 2.0 mm. Fluid pressure should be set at 18-20 psi. Atomization will likely need to be set at the maximum inbound pressure to minimize the large particle size that is inherent when using HVLP guns. Generally, that inbound pressure is approximately 29 psi. Check with your equipment dealer for the maximum inbound pressure rating that can be used. This will allow for the gun to operate at 10 lbs. or less at the air cap to maintain the maximum air pressure at the air cap that is allowed for HVLP guns. This system is the least ideal due to large particle size, lower flow rates, and reduced transfer efficiency. However, this type of set up is the only option for some water base coatings and can result in a satisfactory finish quality.
Other factors for applying water base coatings
Always make sure the coatings are warmed to room temperature before spraying. Be sure that board surface temperatures are at least 72F. In some cases, heating the water base coating to around 92F will eliminate many of the problems typically associated with applying these types of coatings. Check with your coating manufacturer for their temperature recommendations. Slow, constant agitation of the coating can be very helpful to lower viscosity of the material and will reduce the atomization energy needed to spray the coating successfully.
In rare cases, these solutions will not work in certain shop environments. Make sure to try different manufacturers’ equipment to verify the success of different gun and pump technologies and designs before giving up on equipment solutions. If all equipment solutions fail, the only resort is to ask your coating supplier for recommendations for a different coating formulation or a coating additive that may be added to your existing coating to allow it to be applied successfully.
One last recommendation: Don’t give up on going “Green!”
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