A perfect finish is one that is as good as new. In other words, it should not be distinguishable from the original paint. The goal is to make the whole job profitable and worthwhile.
Professionals would tend to agree that time spent in the preparation phase is time well spent especially since the general attitude in the Market is "there's never enough time to do it right from the beginning but always time to do it again? Repeating the job always costs more time and money than it does to do it correctly from the beginning. That is why surface preparation is very important just as important as application if not more.
At the factory assembly line, paint is applied on a smooth and clean surface. The refinisher has to do the same. One can divide the job into different stage:
1. Denting and replacing parts.
2. Putty application and shaping.
3. Priming.
4. App|ying topcoat.
It is imperative that one does not move to a stage until the previous one is completed to perfection. This ensures the proper work-flow and saves time and money.
The 1st job is to identify the general condition of the substrate. Skipping this stage may result in very costly corrective measures, which could include the removal of all the paint new and old.
1. Clean the old surface well.
2. Inspect carefully. Usually horizontal surfaces will be affected first so it is generally wise to start there.
3. 0bserve the gloss level of the finish. A low gloss level may signify microscopic defects.
4. Check for the adhesion. If need be, you may scratch with a knife (close to the repaint area) the old paint and see if it comes off.
5. Determine the type of paint that was used by using the following table as a guideline.
6. The paint is in good condition:
If the existing paint is not reacting to the solvent test and is in good shape, you may spray any type of paint on top of it if you clean it well (use Genclean 8281) blow it with compressed air and wipe it with a tack cloth.
N.B.  Wax and silicone may penetrate underneath the surface and become very hard to detect. That is why, it is sometimes recommended to add a cleaning agent or detergent to the water used during sanding. Pay special attention to spots caused by tar, gasoline, battery acid, antigel, brake fluids. These products may also penetrate underneath the paint film and they have to be sanded away.
7. The paint is in bad condition:
Most defects are gradual and cannot be stopped by simply repainting. In fact repainting usually accelerates the process. This is why the original paint has to be removed and there are two ways to accomplish this task: Using orbital sander or paint remover. Sanders are usually used for panel repairs. You may start with using coarse paper and gradually moving up. Paint removers are usually used for large surfaces (it is usually helpful to make scratches in the paint to accelerate the action of the paint remover).
N.B.  Remove paint on plastics with sanding and on wood with burning. The metal may seem clean and glossy and still have dust that may attack the paint and lower its adhesion. In the workshop, a cleaning rust, traces of oil, grease, pollution, alkali residues, acids, etc. It will also attack the metal and improve the adhesion properties.
Refer to car manufacturer's color code. Use HyMix software to extract formula (make sure to have the right variant shade). Formulate and spray test panel. Adjust if necessary before spraying.
The goal of sanding is two fold:
1. Obtain a smooth surface.
2. Obtain good adhesion between coats.
Generally speaking it is wise to apply a guide coat that helps in determining the areas that have been sanded as opposed to the areas which still require sanding. You can either use a dry guide coat or dilute a contrasting color (9:1) and spray a mist coat (avoid red, maroon and yellow colors). Avoid skipping more than one grade of sandpaper at a time.
3. Use proper masking film or paper. Do not use newspaper.
Spraying technique
Spraying technique has to be learned and constantly practiced.

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