Step 1: Find the nearest community college with an Auto Body program and enroll in it. They will be able to answer half of your questions in the first year.
What you're asking is like asking how to treat an injured person after a serious car crash. There's hundreds of variables. No two injuries, and no two paint jobs are the same. You should be able to find web sites or text books that show how to handle specific situations, but there is no step-by-step procedure that covers every car. Even the manufacturers of paint and other products have their own recommendations, and professionals attend their classes all the time to learn the newest techniques.
Some paint colors are more prone to fading, and matching patches is a challenge. Most paints require additives that make them flexible when they're being used on plastic bumper covers. I've watched two friends do amazing bodywork for years but that is something I do not have the talent for. They use special sealers and primers, and some of the more expensive paint brands. None of that is necessary unless you want to do very high-quality work.
The sanding you're asking about, from what I've observed, is all done by feel. Here again, there is no set procedure. They start out with very course paper to shape the contours or remove old paint, then work up to finer and finer paper. Some primers call for a relatively rough surface finish so they have something to bite into. They Have good fill properties to fill the sanding scratches. Some primers don't fill or build up well and need a very smooth surface. Removing dents is an art too. It's rare for a professional to use plastic body fillers more than 1/16" thick.
With some paints, the final color is affected by the air temperature, the air pressure, how far the spray gun is from the panel, how thick you lay it on, and the humidity. Different final colors of primer will show through as different colors of paint.
Tuesday, December 17th, 2013 AT 8:34 PM