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The surface treatment process

The surface treatment process


Corrosion protection measures are begun as early as in the body shop. Galvanized sheet metal is used for those parts of the car body that are most exposed to corrosive attacks. See also page 800Ä15.


1. Cleaning the body shell from the body shop. When the body shell arrives at the paint shop it goes through an initial coarse cleaning process. Here all the filings, etc. left over from the mechanical machining processes are removed as well as most of the oil and grease that the body has picked up in the body assembly shop.

2. Degreasing To obtain as good a foundation as possible for the subsequent surface treatment, the remaining oil and grease is removed from the body by means of an alkali degreasant.

3. Phosphatizing Phosphatizing gives basic corrosion protection as well as a good foundation for the primer.

4. Rinsing The body is rinsed to flush away the remnants of the phosphatizing process.

5. Passivation A chromiferous solution is sprayed over the body. This densifies the phosphate layer and so further increases corrosion protection efficiency.

6. Rinsing The body is rinsed with desalinized water.

7. Cathodic electrodip (ED) The body is submersed in a bath of anti-corrosion paint and a negative electric charge is applied to the body. The positively charged paint particles are then attracted to the body in the same way as iron filings are attracted to a magnet. A layer of anti-corrosion paint is thus deposited on the entire body surface, including almost inaccessible places in cavities, joints and the like.

8. Rinsing After the ED bath, the body is dipped and rinsed clean of surplus paint particles.

9. Oven drying The paint from the ED bath dries in 17 minutes at a temperature of +182°C.

10. Inspection and grinding Any specks of dust and paint runs are removed.

11. Sealing Seams, folds, joints, etc. are sealed with PVC sealing compound throughout the body. The purpose of this is chiefly to prevent moisture from getting in but the sealant also has a sound- absorbing effect.

12. Sound absorbing material Sound absorbing material is affixed at strategic points in the body to reduce resonant noise.

13. Oven drying The body passes through an IR oven to harden the sealant. The sound absorbing material also softens and shapes itself to the contours of the bodywork.

14. Cleaning Dust is removed from the body.

15. Undersealing Stone damage protection compound and underseal are applied to all bottom pan and wheel arch surfaces.

16. Undercoat The undercoat constitutes a good foundation for the finish paint as regards both adhesion and appearance. In addition, it provides additional corrosion protection in that it affords good protection against stone damage. The paint is applied by means of industrial robots equipped with rotary spray nozzles. When they rotate, a fine mist of paint is formed which migrates to the body and settles on it as an extremely even layer of paint. Inside surfaces which are hard to reach are painted manually.

17. Oven drying The undercoat is oven-dried for 32 minutes at a temperature of +155°C.

18. Grinding Any specks of dust and paint runs rubbed down.

19. Cleaning Grinding dust and other foreign particles are removed from the body so that it is absolutely clean before the finish paint is applied.

20. Finish paint Just like the undercoat, the finish paint is applied by means of industrial robots equipped with rotary spray nozzles. Metallic paint is applied in two layers. First a thin layer with a high pigment content and then a thick layer of clear transparent enamel which protects the pigment and lends a high gloss to the finish paint.

21. Oven drying The finish paint is oven-dried at a temperature of +140°C.

22. Final inspection After painting the body is carefully inspected. If any paint runs, scratches, specks of dust or other imperfections are found, the body is sent to be touched up before proceeding to the next station.

23. Anti-corrosion treatment Penetrating cavity wax is sprayed into the cavities in sills, reinforcement members, etc.

24. Oven The body is heated up to +60°C for 15 minutes so that the cavity wax will spread out as much as possible in the cavities.