Wet Spray Painting
What is Wet Spray painting?
Spray painting is a painting technique where a device sprays a coating (paint, ink, varnish, etc.) through the air onto a surface. The most common types employ compressed gas—usually air—to atomize and direct the paint particles. Spray guns evolved from airbrushes, and the two are usually distinguished by their size and the size of the spray pattern they produce.
Airbrushes are hand-held and used instead of a brush for detailed work such as photo retouching, painting nails or fine art. Air gun spraying uses equipment that is generally larger. It is typically used for covering large surfaces with an even coating of liquid. Spray guns can be either automated or hand-held and have interchangeable heads to allow for different spray patterns.
Wet Spray Painting
The success of the wet spray painting process, like any other painting process, is dependent upon the integrity of the pre-treatment processes as well as the painting process itself. In order to present the components in a fit state for painting, they must first undertake a series of preliminary processes which are outlined in more detail on the subsequent pages.
Briefly the processes are as follows:
- Cleaning to remove all debris and lubricants from the components.
- Rinsing to remove any excess cleaner.
- Phosphating, if required, in order to change the surface and formulate a phosphate film on the surface which has the ideal grain configuration for painting.
- Rinsing to remove excess phosphate.
- The wet spray painting operation itself.
- Stoving through a gas fired oven.
Cleaning
In order to remove all debris, oils and contaminants, a series of different cleaning methods are used. These vary from a low strength ‘knock-off’ cleaner to a highly caustic powder cleaner using the electrophoretic process line; two bespoke cleaning machines one of which has the added advantage of a drying unit or using the iron phosphate machine which has a mild acidic cleaner in the iron phosphate solution. All these methods and cleaners are effective on a wide variety of oils and greases.
Experience tells which cleaning method to use from the mild acidic cleaner in the iron phosphate to the stronger caustic cleaner in the cleaning machines and finally the heavy caustic cleaners on the electrophoretic line. The high alkalinity of the electrophoretic cleaners helps shift some types of soil although the level of caustic is not sufficient to de-rust effectively.
Phosphating
There are two types of phosphate available namely iron phosphate or zinc phosphate as below:
Iron Phosphating
Iron phosphate deposits a fine crystalline phosphate surface at around 0.2-0.4g/M². The phosphate is applied by a spray process through a high pressure in-line spray production line which has the added advantage of cleaning the parts as well as applying the phosphate.
The chemical used is a liquid material which is a combined coater (heavier coating weight phosphate) and cleaner solution designed to simultaneously degrease and pre-treat ferrous substrates prior to painting.
Zinc Phosphating
Trication Zinc Phosphate deposits a fine crystalline zinc phosphate at around 1 -3g/ M². The Trication formula comprises a mixture of zinc, manganese and nickel to make up the phosphate structure. The Nickel content is expressly excluded from the substances of concern list in the ‘End of Vehicle Life Directive – 200/53/EC’ This is due to the fact that currently there is no better way to phosphate automotive parts.
‘Tricats’ generally ‘sludge’ less than traditional calcium modified zinc phosphates and are favored by Honda, amongst others. Keeping the temperature low, if possible, helps minimise ‘mapping’ problems from uneven phosphate build up.
This process is in accordance with BS 3189 Type 4 and DEF STAN 03 – 11 Class IV
Special points of interest
• Salt spray resistance to a 500 hours as per ISO 7253
• Adhesion in accordance with ISO2409 to GtO
• Scratch test in accordance with ISO1518 using a load of 1500g
• Pencil hardness in accordance with ASTM D3363-92A at 3H
• Impact Resistance (direct/reverse) in accordance with ISO3900 to > 2.5 N.m.
• High resistance to chemical agent attack including all common fuels.
• High resistance to the effects of weathering as shown by testing to BS3900: Part F16
Rinsing
It is absolutely imperative that any excess zinc phosphate solution is removed prior to further processing. Consequently, following on from the zinc phosphate tanks are two rinse tanks similar to those following on from the cleaning tanks. Again, these tanks are filled with cold town’s water agitated by passing air through the water. The ninth tank has a steady flow of water feeding into the tank which subsequently feeds into the eighth tank. By using this method the ninth tank is kept clean and free from contamination
Sealing
This solution is a chromium-free, inorganic-based post rinse, formulated for use after the zinc phosphating pre-treatment process and prior to painting.
Its use will materially improve the overall system performance. This solution can either be used as a spray or immersion system.
Wet Spray Painting
Having first thoroughly cleaned the parts, and if necessary phosphated the surface, the parts are loaded onto hooks ready for painting. In some instances, bespoke jigs are used, in conjunction with a rotating work table, which allows for masking of those areas which are required to be paint free. The parts are then sprayed manually using an air powered spray gun. The painting options are either an etch primer which is more functional than decorative; a one-coat stoving enamel which is both functional and decorative or a two-part paint finish comprising a primer coat giving good adhesion and protection followed by a top coat giving a smooth, functional, decorative finish.
This last option allows for using paints with special properties such as infra-red reflectivity and chemical attack resistance. Having satisfactorily coated the parts they are transferred to the conveyor in order to be cured. Having passed through the curing oven, the parts are unloaded from the track, visibly inspected for defects and packed to the customers’ requirements.
