Basically what you're looking for is a good, solid foundation of existing paint that is stable and still strongly bonded to the car. Many times an original factory paint job, even after decades will meet these criteria. Before you start painting, it is a good idea to keep an eye on the weather for rain or snow. You don't want to start painting only to stop abruptly because of the weather. Knock down texture is basically a orange peel that is sprayed on and allowed to "set-up" for a minute or two then gently "knocked down" by running a 6" drywall knife over it, "knocking down" the surface of the texture. Although knock down is a flattened version of orange peel, I suggest you do not use a spray can of orange peel, the knock down comes in its own spray can and the results are much better, again experiment before spraying it on the wall. However, If ever you apply any of the textures to the wall and are not satisfied with the results, wash the texture off immediately before it dries, then allow the wall to dry and try it again. Once you have the texture applied to your liking and you have allowed it to dry, you may want to paint the patches a couple of time before painting the entire wall, especially for walls where the paint to be applied will have some sheen. Multiple coats on these areas will make them blend in, unnoticeable, with the rest of the wall. You will know if the patches need more coats of paint, because the patch/texture will have absorbed the paint causing the spot to look duller than the rest of the existing wall.
Tools, there is a wide variety of brushes and rollers to choose from, it is critical to select quality tools and maintain them. We will discuss brushes first, I use and recommend only Purdy brand brushes, they will give you the best results in terms of coverage and make the project go much easier and quicker. A Purdy brush is a little more expensive but if kept clean promptly after each use and stored properly, it will last for years and many projects. Most any other brushes out there, especially the cheap ones, are made of far inferior hair and I liken them to using a hand (or whisk) broom to paint with. Using lower quality tools will only frustrate you and lead you to assume that you are incapable of painting. You are capable! and the proper tools, materials, and advise are all you need. If your painting project will be on going for a couple of days, you can wet the brush with paint, wrap the brush up in a plastic sandwich bag and tape the plastic bag to the handle above the metal ferrule. Sealing it up will keep it from drying out, storing it flat will maintain the shape and permit you to use it for a couple of days without cleaning it. Because, after a couple of days or if it has had extended use on a hot day, it will loose its shape and start to collect a lot of dried paint on the hair, you should clean it with warm water and a wire brush. Once you have ran some water through the brush hair, the wire brush will then be used to remove the dried paint from the hair. Always run the wire brush down the entire length of the hair and never across it, to prevent crimping and damaging the hair. After you get the brush clean, spin it between your two hands, shedding as much water as possible from it. Then, using the wire brush again, comb the hair, shape/style the hair with your hands and lay it flat to dry. Once dry, if you will not be using the paint brush again soon, protect it by storing it in the paper shuck/cover that it came in and it will be like new the next time you use it. Use Tinted Primer for Dark Paint Colors.In our second case, suppose you want to apply a deep, dark blue to a beige wall. The good news is that it will be easier to darken a light color than it was to lighten a dark color. In fact, it may only take 2 - 4 coats to get total color coverage in this scenario. The bad news is that if you start out with a white primer you are already moving in the wrong direction. Doing so could increase the number of paint coats you need to 3 - 6... plus the coat of primer! That's a lot of painting! If a desired shade is not obtainable in custom-or ready-mixed paints, white paints may be tinted with colors-in-oil. To do this, mix the color-in-oil with a small amount of turpentine or mineral spirits and stir this into the white paint, a little at a time. If a blended color is desired, more than one color may be added, such as a chrome green and chrome yellow pigments to produce a lettuce green shade. Bubbles in the existing paint are sure indications of rust below. These areas will require extra special attention. As you remove the bubbled paint you'll inevitably find the cancer goes much further then it appeared on the surface. And unless you cut all the rust out it's going to come back. There are chemical rust converters on the market, and some do a very good job. But understand that none will probably encapsulate rust forever. Be honest with yourself about how long you expect this paint job to last. If you expect to repaint the car in 5 years and you're on an extremely tight budget, then rust converters may be the approach to take.