The body panels inside and out all have a factory applied primer (Electro-deposition process) and the inside of the doors and other areas were then sprayed with a product that is generally called a wax. Yes it does contain wax but it also contains some other chemicals. The wax part is excellent at keeping moisture away from the base metal that the panels are made of. When you combine the primer and the wax you have the best corrosion protection that you can get for a reasonable cost. A company called "Wurth" from Germany sells a very good wax that you can buy at an Auto Paint Supply Store. It is usually sprayed on with a special spray gun that has all different attachment wands so that you can get inside of rocker panels and hidden areas of the body. It is a recommended procedure that you do this any time you weld panels together so that the inside won't immediately start to rust. When you weld you usually remove the corrosion protection because of the heat of the welding. You can put it on with a brush or pour it on too.
What is Basecoat Clearcoat ?
Basecoat Clearcoat Systems started to be used in OEM applications in the 1980's. There were many problems in the beginning that have now been solved with this system. Lincoln's were one of the first cars to be painted with this system.
Most cars now are painted with a basecoat clearcoat system. It is very durable. Car manufacturers want their cars to look good for as long as possible so that they keep their value. The "Glamour" part of paint is what people see when they look at a car. It does not include any of the other properties that paints are designed to have. "Glamour" includes Color, Metallic Effect and Gloss. In the past all of these were combined in the "Topcoat". Now they are applied in separate layers.
The Color in paint is obtained with pigments. These are usually natural minerals or ingredients that can eventually deteriorate.
The Metallic effect is caused by aluminum(or other) flakes of a certain size for a certain effect, Coarse or Fine. These can also deteriorate.
The Basecoat Clearcoat system puts a layer of protection over these two ingredients. The Color and Metallic are added together and applied. The Clearcoat contains all the chemicals to protect the topcoat from sun, salt, water, scratching etc. Instead of trying to add the protective chemicals to the pigments and aluminum flakes, the paint companies can now just put a protective, glossy layer over the basecoat and simplify the process and also make it more durable. Even solid color cars are painted this way. If you compound the topcoat of a car and color comes off then it is not a basecoat clearcoat system. Try not to remove any of the clearcoat because it is the only thing that protects your paint job. Most metallic colored Merkur's were painted with this system. The solid color cars that I have seen are not clearcoated.
Spraying the car
The OEM paint procedures have been a very protected process and are very complex as far as specifics but I will start by giving you a general idea of what is involved.
First some general terms:
Undercoats: Prevent rust, provide foundation for topcoat.
Topcoats: Level surface, provide glossy appearance which is extremely important to the new car buyer.
Step 1. The body is assembled at the body plant. It's not all the same type of steel nor is it all bare steel. Sometimes the doors are made at a different shop and may already be treated since they may not be attached for a couple of days.
Step 2. The greasy dirty body with welding slag and splatter on it goes to the paint shop. Hand inspection with gloves and a very brightly lit area (1-2,000 lux?) is done to give final OK and sometimes sanded to remove defects before next step, 11 step process; hot water wash, hot water wash,degreaser, shower, shower, dip into zinc formula with electro-deposition method, shower, shower, shower, final rinse,drying at 212-284 degrees F.
Step 3. A surfacer is applied (dip-tank) with high voltage DC (cationic electro-deposition) and dried at 300-340 F to level the zinc-phosphate surface. The thick rubberized under coating is then applied and dried.
Step 4. Fit is checked of body panels. all seam sealers and sound deadeners are applied. Color coat is applied. Body is baked at 300+ degrees for 15-30 minutes. Check for defects and fix.
Step 5. Clean body and apply second top coat. Check for defects or send to baking and cooling stage. If there are major defects the body is sanded and sent back through the final paint coat line.
Step 6. Blackout areas are done. Pinstriping is done. Rocker panels sprayed if different color. Touch up and polish if necessary. All other parts are sprayed with completely different processes.
As you can see this is a very expensive process and very complicated. To get a perfect paint job without all this equipment is not easy but it is possible.