Underbody Coating

Aim:
About Under-Body Coating, its types in detail and its pros and cons.

Underbody coating:

Underbody coating provides long term protection from corrosion to car’s underbody. It also protects parts like internal body panels, frame rails and other inner cavities that are not physically accessible but are prone to corrosion.

Modern Automotive Coating Processes:

The modern automotive coating consists of five steps:

• Pre-treatment: removes and cleans excess metal and forms an appropriate surface structure enabling bonding of a corrosion protection layer.
• The next step is electrodeposition (ED) of the anti-corrosion or rust prevention layer.
• A sealer like Poly Vinyl Chloride (PVC) is applied for anti-corrosion, elimination of water leaks, and minimization of chipping and vibrational noise.
• A primer is then applied to promote adhesion between the surface and the basecoat; it also imparts a smoother surface for subsequent layers and has anti-chipping properties.
• Finally, the topcoats that include a basecoat and clear coat are applied; they provide surface properties that are sought after, including colour, appearance, glass, smoothness, and weather resistance.

Flow of body coating in automotive

Pre-treatment:

The first step in preparing BIW for subsequent coating is the pre-treatment process. Pre-treatment consists of cleaning the body surface to remove remaining oils from the stamping process and welding residues through three primary liquid dip processes of degreasing, conditioning and phosphating. The degreasing solution is composed of alkaline salts and surfactants and can include caustic soda, tri-sodium phosphate, and sodium carbonate. The surfactants are types of detergents for emulsifying oils and lubricants on the BIW.

Pre-treatment sequence of Biw

Electrodeposition (ED):

The metal underbody and frames of automobiles are coated to prevent corrosion, whereas other areas like the roof are not rust-proofed. When not coated to prevent corrosion, the structural metals (or other materials) are primed before applying additional coatings. The ED processes are modified as per the BIW material property. This ED coating provides rust and corrosion protection.

Electrodeposition coating process

Rust-proof materials: Sealer/PVC

The third step is underbody coating & seam sealing using PVC. Figure displays the lower door sections & rocker panel locations where PVC or urethanes are applied as an anti-chipping protection, & shows the cross section of this layer with respect to the other coatings on the body parts.

During this third step, it is also typical to apply a soft tip primer coat that improves the chipping resistance. This layer is applied to the front edge of the hood, which is an area prone to chipping shock, using a high elastic resin that resides between the ED & primer coats. Also, during the third step, the radiator supports, wheel housing, & under areas of the body are coated with a relatively dull black pigment called a blackout coating.

Underbody Coating

Primer:

The fourth coating step is the application of a primer surface or simply primer. It can be water-borne, solvent-borne or a powder. These primers are used to improve adhesion between the E-coat and topcoat, provide chipping protection, enhance paint appearance and also be environmentally compatible with emission regulations.

Top-coat:

The final step in the body coating process is to apply the topcoat, which consists of two layers—the basecoat and clear coat. The basecoat contains the primary colouring pigment and the clear coat provides a protective coating against environmental effects, corrosion, and UV light degradation promotes unmatched colour retention and provides a smooth unblemished even finish.

Types of Underbody coating:

Rubberized Underbody Coating:

Ingredients used in the manufacturing of undercoating include fiberglass, ceramics, rubber, asphalt, and silicone. Each has different properties in terms of heat resistance and retention, but all give protection against the elements and damage.

This is generally thought of as the best underbody coating for your car. It preserves the underbody well, protecting it from scratches. Applying this to the undercarriage is not much of a hassle either.

Wax Based Underbody Coating:

This – more or less – ephemeral undercoating will wear away in a few months’ time. On the upside, wax-based undercoating puts up a big fight against moisture. It is also the most affordable option out there.

Asphalt Based Automotive Undercoating:

Best suited for lorries, trucks and large vehicles this provides a greater degree of protection compared to rubber-based coatings. However, asphalt-based coatings contain little amounts of rubber for sound-insulation purposes. The process of applying this coating is a bit time-consuming though.

Polyurethane Underbody Coating:

This coating closes up splits and fractures on the underbody of the vehicle. It offers up good protection against corrosion and also bonds with the metal. Unlike in the case of asphalt-based coating, you can paint over the polyurethane coating.

Undercoating Pros:

Rust Prevention: Underbody coating ideally should be applied when the car comes out new from the factory, when there are no dust particles present. This will result in a good adhesive coating to prevent rusting of the underbody surface and also extending the life of the parts. It becomes extremely important to apply this coating when the vehicle is to be driven close to coastal areas.

Protection against debris: It is one of the primary reasons for applying an underbody coating. During its lifetime, a vehicle travels over debris the entire time, and this is especially true for Indian roads. These debris fly off from the tyres and hit the underbody, and can possibly damage the underbody components which can get expensive. By giving a sufficient coating the life of the sufrace can be increased.

Sound reduction: Applying thick layers of covering can reduce the sound within the cabin upto a certain degree. Rubberized paint can reduce the noise from the tyres, the road and the debris.

Easier maintenance of the vehicle: By giving a uniform coating, the underbody is well protected from moisture, which means that for cleaning the underbody water can be used. A good coating can ensure that water does not enter the components and start rusting prematurely. The coating also prevents retention of water within the parts, which can lead to creation of slush.

Undercoating Cons:

High cost: The cost of underbody coating has reduced over the years, but it is still expensive. The option of applying a coat at the dealership can get quite expensive, since dealerships will look forward to making profits and hence increasing the price for an underbody coating.

Damage during application: In case of thick rubberized paints, if the coating is not applied properly, then there is a possibility of the paint getting into moving parts and create mechanical failures. It can affect the differential, propeller shafts, joints and even the suspension. If not done carefully, there is also the possibilty of the coat damaging the paint job at the lower edges of the car.

Added weight: Although insignificant, there is a possibility that the extra weight can affect the fuel economy of the vehicle.