A question I frequently see is, what sort of paint system should I use? Base/Clear? Base only? Urethane? I will let you in on a little secret, it really doesn't matter, within reason. What makes great paint look great has almost nothing to do with the paint but rather the panel preparation prior to the paint, and the sanding and polishing after its painted.
While the terminology is confusing -- Single Stage, Two Stage, Catalyzed, Urethane, Acrylic, Enamel, Lacquer, POR15, etc... the choices are quite simple. In modern paint there really is only a couple choices you need to make. I will explain, perhaps with too much detail.
I will not be offended if you jump to the end to read the bottom line.
The Nitty Gritty
Paint comes in four main flavors: Nitrocellulose Lacquer, Urethanes, Acrylic, and Epoxy. Most paints are available either as single or dual stage and catalyzed or not. Note, frequently the word enamel will follow the paint type (ie Acrylic Enamel), this is almost meaningless. Enamel is defined as a paint that dries to a hard glossy finish, hardly helpfull!
Some paints are available as either single or dual(double) stage paint. A single stage paint is a paint where the top coat is tinted. Dual stage paint has its pigment in the base coat, and is covered with a clear coat of paint. Not surpizingly, dual stage paint is also referred to as base/clear. In most circumstances, people are referring to Urethanes paints when talking about base/clear.
In dual stage urethane paint systems, a basecoat is applied, This coat is generally not shiny, and may or may not contain metallic or pearl flakes. It is applied extremely thin (1-3mils) and topcoated (6-12 mils) before it has dried with a high-gloss, clear urethane. One looks through the clear and sees the color in the basecoat. When done properly, it is has a high luster and is very durable. If you are using metallics or pearls, it is the only system.
Single stage urethane is simply dual stage paint where the clearcoat has been tinted and the basecoat has not been applied. Rather than using a clear coat, the clear is tinted with solid pigments. The paint is virtually identical other than being opaque rather than clear. In older systems, the single stage was not as vibrant as the dual stage, but that is no longer the case. Single stage paint is slightly less expensive than dual stage because of the need for less material.
One of the main advantages of dual stage is that it is easily repaired by a body shop. After a body repair, you can spot in a small area with more color, mist the panel with color, and then reclear the area. The reclearing makes it difficult to detect the seem between old paint and new paint, as the seam is clear. Repairing single stage is the same, but it is easier to see the seam between the old paint and the new paint, particularly if the color is even a tiny bit off. For this reason, and this reason alone, I prefer dual stage paint.
Most bodyshops only have dual stage paint mixing systems on site. For this reason and the ease of repair, most bodyshops prefer dual stage paint.
Catalyzed paints use a chemical reaction to cure, Non-Catalyzed paints air dry. Catalyzed paints are more expensive and more durable than non-catalyzed paints. Catalyzed paint need a base paint, a hardner, and a reducer (thinner) and must be mixed just prior to using. Non-Catalyzed paints do not require mixing, and will remain liquid a long time if not exposed to air. All spray paints are Non-Catalyzed. All body shops use catalyzed paints.
Etching or Direct to Metal
Some paints are direct to metal, some are not and require a primer. DTM paints have a small amount of acid in them to etch the metal they are applied to form a chemical bond to the underlying metal. Make sure you know which you have. If you spray a non DTM paint directly on metal, it will flake off, and conversely some DTM paints will blister most primers. Self etching and DTM are the same thing.
Paint Brands and Lines
Every paint brand (Sherwin Williams, PPG, Glasurit, etc) creates excellent paint nowadays. The chemistry is well known, and all major Urethanes are excellent. There are differences in approach, and how easy they are to spray, and mix, and other things the painter needs to worry about. But, the car owner will probably never be able to tell the difference from one paint brand to another.
Inside the brands, there are paint lines. Sherwin has Sherwin Williams line, the Dimension Line, and the Western Refinisher's Line. The chemistry of the paints is pretty much the same, but not all the colors are.
For example: You cannot buy Ferrari Rosso Corso in anything but the most expensive Sherwin Williams line. The Western line only has a few dozen colors in total you can buy. If you don't care whether you have the exact color match, you can save a mint choosing a fleet paint color. Can you tell the difference between Rosso Corso (Sherwin only ~$450/gallon), Miata Red (Sherwin ~$400/gallon, Dimension ~$200/gallon) and fleet red (~$400/gallon, Dimension ~$200/gallon, Western ~$100/gallon)?
Types of paint
The old school finish, frequently referred to as simply "Lacquer". It is a pigmented cellose-base in a disolved in mixture of solvent. In other words, color (pigment) suspended in lacquer thinner. This stuff is gorgeous, super cheap, easy to spray, fast drying, short lasting, labor intensive, and illegal almost everywhere. People would spray on coat after coat of this stuff, hand rubbing between coats, to build up a deep luscious color (this is where the still persistent myth of multiple coats comes from). Unfortunately, this paint did not age well and needed to stay out of the sun and elements to remain colorful. Since you probably can't buy this stuff, my advice is not really relevant, but I would not use this paint in favor of the new paints.
All modern cars are using Urethanes. These are crystalline compounds that are either tinted or clear. They are quite hard and flexible compared to other paints and stand up to the elements exceptionally well. They are wildly expensive ($200 to $500 a gallon) and difficult to apply well, but last a long time. In most places, this is the only type of paint you will be able to get for your car, and really the only one worth considering. They are all catalyzed and available a single stage or dual stage. They are not direct to metal and require a primer.
Acryclic paint is made out of acrylic resin. It is cheap, easy to apply, and relatively long lasting. It is not as durable as urethane, much more durable than nitrocellulose lacquer, and suitable for many applications. It does not have the luster of urethane, either. It is available in catalyzed and non catalyzed version. They can be either Direct To Metal or not.
Almost all spray paints are Acrylic as well as most of Eastwood's paints.
I use acrylic paint a lot in restoration work. I use it on almost all non exterior surfaces (air cleaners, shocks, pulleys, interior trim) and some exterior ones (wheels for example). It is cheap to get color matched, holds up well, and sprays well out of any spray gun, including Preval Sprayers (little spraycans with jars so you can mix your own paint http://www.prevalspraygun.com/
). I almost always use a catalyzed, non-direct-to-metal version since it is more durable. But, occasionally I use SEM's Trim Black spray paints as well. They are direct to metal, inexpensive, and durable.
Most chassis paints and rust converters (POR15) are an epoxy base. Epoxy paints are extremely strong and durable, low gloss, easy to apply and moderately expensive. They come in Catalzyed and non Catalyzed versions. I use them on chassis components and places that have some surface rust I cannot remove for whatever reason.
A note on Rust Converters
Rust converters chemically bond to iron oxide (rust) making a new compound that is extremely hard, durable, and waterproof and airtight. They rely on the iron oxide for their adhesion. They do not prevent rust, they only bind to rust.
Self-etching primers etch themselves into steel, forming a waterproof airtight barrier, but they do not bite into iron oxide.
What does this mean? If you cannot remove all rust from a piece of metal, use POR15 by all means. But if you have a rust free piece of steel, do not use POR15 on it, use an etching primer, you will get a stronger, more durable finish for less money that way.
The bottom line
If you are taking your car to a shop to have its exterior painted, besides color choice, you really have to worry about the choice between single stage or dual stage (base/clear). Both are excellent choices. Single stage is probably, but not necessarily a little bit cheaper than dual stage, but harder to repair invisibly by a professional or a skilled amateur. When done well, you will not be able to tell the difference in material choice by looking at it.
Generally, I would let the shop pick the materials they want to use. They may have a better deal from one vendor than another or just like the way a paint flows out of their favorite paint gun (don't even get me started on paint guns, they probably have more to do with the final finish then the finish!) and get better results with that. Chose your bodyman carefully and trust them.
Most people confuse the quality of bodywork with the quality of the paint. Look at the shops previous work. Get references. Really. Make sure the paint holds up a year or two after it left the shop. If it didn't it wasn't the paint, it was the painter. There is a tremendous amount of skill in body and paint work. As in most things in life, the people make the difference, not the chemicals.
1974 Alfa Spider Daily Driver
1965 Jaguar E-Type
1977 Ferrari 308 GTB
1965 Plymouth Sport Fury with a 426 wedge