How to apply the basic coating process to the automobile sheet metal parts?

2019-12-24 10:23:50

In fact, the basic coating process is widely used in the whole process of body sheet metal repair. According to the author's years of experience in automobile sheet metal repair, the specific application of basic coating process in the repair and replacement of body sheet metal parts can be summarized into the following categories:

I. Basic coating of ferrous metal plate

Ferrous metal plate is easy to be oxidized and rusted gradually due to contact with air or contamination with harmful substances and other factors, resulting in yellow rust spots and damage to the original coating layer. Especially for the parts processed by hammering, welding and other processes in the process of sheet metal operation, it is more likely to damage the original coating protection layer and accelerate corrosion. Even if the paint film is sprayed, the paint film will fall off together with the basic coating quickly. At this time, a thorough coating treatment must be carried out again.

Use wire brush or iron sand paper to polish and clean the surface of ferrous metal plate. If possible, it can use the cleaning tool, nylon grinding plate, which has been widely used at present, to thoroughly polish and clean the metal surface. When disassembling and repairing the car body (especially the accident car), the folding and overlapping parts of the plate material shall be sprayed with a spray liquid specially used for rust removal, paint removal, and removal of oxide slag and dirt before and after metal welding, or replaced with a rust proof primer with good rust prevention ability.

II. Basic coating of joint surface of sheet metal parts

If the automobile body after repair is not treated with anti rust treatment, it will be oxidized and rusted due to the existence of various impurities and water vapor in the joint of sheet metal parts within one or two years, which will affect the strength of welding points, so the welding zinc agent shall be sprayed in time.

Welding zinc agent (zinc rich coating) is a new material specially used for anti-oxidation and corrosion of the joint surface of sheet metal parts. It has good conductivity, high temperature resistance and electrochemical cathodic protection. It can form a layer of antirust protection layer between the welding joint surfaces of sheet metal parts, improve the ability of the parts around the welding joints not to be rusted, protect the welding joints, and ensure that the strength of the repaired body is no less than the original body.

III. basic coating of plastic welding

Although the surface of the weld, welding spot and repaired steel plate has been polished and cleaned, there are still numerous micro invasive and irregular small bumps, which are invisible to the naked eye, including numerous oxides, impurities, water vapor, etc. Although ordinary putty or antirust paint has antirust function, it can only prevent the invasion of surface rust and play a temporary sealing role. As long as affected by humidity and other harsh environment, defects will appear soon. If the epoxy primer is used, it can play an effective protective role.

Epoxy primer is a good rust inhibitor, which can be applied to the seam of panel or lap joint after welding, grinding, heating, quenching and contraction. Generally, the feather edge of the paint film on the edge of the repair part (i.e. the groove of the paint film) is well prepared, and the epoxy primer is applied after the surface is cleaned, so as to ensure that the panel after various treatments will not rust for a long time, even if there is rust in the original, it will prevent the expansion of the rust range.

IV. foundation coating at joints

For the metal plate directly coated without special sheet metal sealing treatment, the seam outside the welding or overlapping joint surface will be deformed and cracked due to the erosion of various sundries, such as oil, coolant, sewage, etc. and the vibration of uneven road surface. The anti rust sealing line of the interface will be broken through and the function will be lost. It is an inevitable process to apply special sheet metal sealant to the joints of plates.

Sheet metal sealant is a non corrosive polyurethane PU Structural adhesive, which can be used for caulking, covering, bonding and sealing. Sometimes it can also replace welding without damaging the paint. It has strong adhesion, high strength after curing, strong and reliable, can enhance the strength of single component and thin shell body, and can maintain the anti-corrosion function of welding or lap joint, especially inside and outside of wheel arch, chassis, rear compartment, bottom plate in cockpit and engine compartment.

V. foundation coating in the chamber of box section components

During the body overhaul, it will be found that the original sheet metal parts in the chamber of box section components have been repaired due to accident shaping before, and the rust spots, steel plates become thinner and penetrated, which is caused by the neglect of internal rust prevention, so that the rust seeps out and expands continuously. These cross-section components are often the main load components of the integrated body, which will have an important impact on the crashworthiness and durability of the vehicle. Therefore, anti rust treatment must be done for the box section or the cavity of bag shaped components.

Anti rust protection wax inside the car body is a good assistant for cavity protection. It has excellent permeability and can easily penetrate into all corners and joints. It is moisture resistant and drainage resistant, which can prevent the cavity that can not be sprayed with paint from rusting and wearing, and prevent the internal rusting of the welding part of the car body plate. In addition, when coating the inner surface of these plates, special spray bar and nozzle can also be used for coating treatment. Ensure the final maintenance quality.

Vi. other functions of body foundation coating

1. Shockproof treatment at the joint of framework and skin

After the engine cover and luggage compartment cover are disassembled, the body sealant must be filled between the skin and the inner lining before closing. This can make the closed skin cling to the lining, otherwise the skin will be loose, there will be abnormal noise when opening or closing the engine cover or luggage compartment cover, and resonance sound may also be generated during driving.

2. Heat insulation, moisture-proof and noise proof treatment of underbody

On the upper surface of the underbody, the metal surfaces of the sheet metal components and welding parts must be polished and painted with antirust primer