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.
Special Surfaces. WHITEWASH. Whitewashes and lime paints must be thin when applied. In fact, best results will be obtained if the application is so thin that the surface to which it is applied may easily be seen through the film while it is wet. The coating will dry opaque, but two thin Coats will give better results than one thick coat.
Are the contours rough or concealed?
Cement-water paints are best suited for application to basement walls which are damp as a result of leakage or condensation. To apply these paints, the same procedure should be followed as is described here for painting exterior masonry walls.
Analyze your existing paint. The first step before determining if a budget paint job will work on your Mustang is to assess the current condition of the paint. In many cases you can prep and paint directly over an existing paint job, but only if it's in solid shape.
Step four: the key to a finished looking painting is to build it just like a house...and by that I mean layers. Paint it in the reverse order in which the eye sees it to make it three-dimensional. By this I mean paint what's farthest from the eye first, and build layer upon layer towards the eye. In other words, do the background first because it should be the farthest from the eye, then add the objects on top of that, and then add the shadows to complete the look. Ultimately it's common sense. If you paint a bowl of fruit, the bowl and fruit need to sit on top of that background, much as it would in real life.
Usually on exposed air ducts of galvanized metal a primer coat of zinc dust-zinc oxide paint is used, before the finish coat is applied.