If you are repairing your vehicle with new OEM or Aftermarket Parts you will purchase them from your supplier with a thin black Enamel Primer that is used primarily for shipping and storage to protect the part from oxidation. This primer is really not good enough to stand the test of time so technicians must add a primer sealer to the interior surfaces to protect it from rust.
Although there are specific guidelines that will help you restore the vehicle to OEM Specifications there are some basic steps that are going to be common across all brands and types of vehicles.
The first thing would be that all surfaces of your replacement parts must be treated with a rust preventative primer on the body of the part and any areas that are to be welded should be treated with a zinc based weld through primer after they are cleaned to bare metal.
The Technician should first fit the part to the vehicle by removing the old part and following sectioning guidelines if not using OEM weld points. For parts like door skins the door shell will also need to be prepped and weld through primer painted on all spots where metal has been exposed or welding will occur.
On the new part you can then begin preparing the panel by using a small belt sander or grinding disk to expose the bare metal where spot welds will be made. Often it is a good idea to center punch each spot location so you can tell where the part has been sanded to bare metal after it has primer replaced.
You can now use a P600g sand paper or scuff pad to prepare the entire inner surface of the panel. The idea is to remove the gloss of the black E Coat Primer that was used in shipping.
You should now use an Ammonia Free Cleaner to wash the interior parts of the panel that must be painted with OEM Primer before they are welded to the vehicle. Ammonia Free Window Cleaner is often used as it is cheap and effective.
First paint all of the areas that must be welded with your Zinc Weld Through Primer. The spot weld and stitch weld areas and not the holes you may have made for plug welds. At the same time you want to paint all the areas on the vehicle that were prepared for welding or had spot welds ground down when the part was removed.
You can now apply your OEM Color Matched E Coat Primer on the interior area of the replacement part. Take care to not spray over the weld through primer that you applied in the previous step.
Wait the recommended time of about 20 to 30 minutes before you continue and add your sound deadening materials to the panel and weld the part in place.
E Coat Primer comes in a variety of OEM Colors and it is recommended that you match the OEM color when installing your new part. Since it is a synthetic enamel it is easiest to just keep a range of colors on hand in spray cans for quick selection and application.
Final Note
OEM repair recommendations are normally the minimum standard when repairing a vehicle. Just as it is important to follow guides for sectioning parts and replacing parts it is also important to prepare parts with rust preventive coatings prior to replacing them.
E Coat Primer Sealer is a minimal standard of protection. If you were restoring a Classic Vehicle you might want to go farther and coat it with an acrylic or urethane enamel for added protection.
Final steps will include sealing all spot weld seams with a seam sealer and in many cases a wax based cavity rust preventative material will need to be applied to the inside of the panels on the interior side of the spot welded seam.
Remember that some panels like door shells will have weep holes for water that might get into the door from rain. Don’t seal off those weep holes.