All paints — whether used for houses, furniture or automobiles — including car restoration projects — consist of pigments, binders, solvents and additives. Under the paint itself is a primer and sometimes a surfacer. We're going to address this article to automotive paints, of course, so let's examine each.
Binders are the major component of all paints, with lacquers, enamels and two-part enamels representing the most common. Lacquer can be nitrocellulous (hardly any is used anymore) or acrylic based. Ordinary enamel can be alkyd or acrylic and two-part enamels can be epoxy or polyurethane or acrylic urethane. The binder is made of resin, with melamine being the most common at the factory level.
Added to the melamine is a drying oil (linseed oil, for instance) or a plastic such as polyurethane or polyvinyl chloride. The sole function of the binder is to keep the pigment in even suspension so that the car's finish is smooth.
Pigment is the color. It is composed of microscopic particles of coloring agents made from an endless number of materials. It is by far the most expensive part of the paint, accounting for at least 50% of the cost and sometimes as much as 80% (reds and purples, for instance).
Pigments can be solids, metallics and tri-coats. Solids are just that: one uniform color is suspended throughout the paint. Solid colors look the same when viewed from any angle. Metallics consist of fine particles of aluminum, mica or other compounds, mixed into the color. These particles create the glitter effect that makes the paints so popular. Painting with metallics, however, is much harder to do than with solids.
Tri-coats are usually called "pearls." These finishes have three separate layers, consisting of a color coat, pearl coat and clear coat. The pearl coat is a transparent paint with fine mica chips mixed in. Pearl coats are very difficult to apply.
Solvents are the thinning agents used in paints to allow it to be sprayed properly. Lacquer solvents are called thinners and enamel solvents are called reducers. Both evaporate at a pre-determined rate depending upon temperature and humidity conditions.
Additives are compounds used to promote special properties, such as faster curing, greater gloss, resistance to silicones, flexibility, etc. Very small amounts are used in each mixture.
Primer is a dull-looking paint applied to the bare metal. It is necessary because the paint (top coat) doesn't really stick very well to bare metal. The top coat does stick well to the primer, though. There are two general types of primers:
Etching Primer is a primer that chemically etches into the metal to provide an exceptionally strong bond. The etching agent in the primer is usually phosphoric acid. Two-part etching primers (epoxy primers) are now used extensively. They cure chemically instead of by evaporation and tend to shrink less than regular primer. Etching primers have great anti-rust properties.
Electrostatic Primer is what is used at the factory. The bare metal body is dipped into a tank of liquid primer and an electric current is sent through to deposit the primer into the molecular surface structure of the metal. After coating, the bodies are cured at 350 degrees F. It is the most effective type of primer, but is virtually impossible to do in shops by hobbyists.
Zinc Chromate Primers are particularly useful when priming different types of metal that will be joined together, such as aluminum and steel. The zinc chromate acts as an insulation barrier against galvanic action and has excellent rust-prevention properties.
Anti-chip Primers are special formulations that cushion the topcoat to prevent chipping. They are most often used along rocker panels and wheel wells. Some of these primers can't be applied over bare metal, so reading directions is very important.
Surfacers are special primers that are formulated to be applied in a thick layer that bonds with the primer coat. They are very effective in filling scratches and other surface irregularities and can be sanded to a very smooth surface that enhances the top coat.
Paint chemists have worked over the years to eliminate the flaws and defects that used to plague the finishes on cars. They have done so with chemical additives.
Flex Agents are added to promote a greater degree of flexibility in the paint to eliminate cracking of the surface. It is used most often in paints sprayed over soft bumpers, beading between fenders, flexible fascia parts and the like.
Fish-Eye additives are used in nearly all finish applications. Unfortunately, silicones and oils travel in the air as vapors and are present in any garage or shop. They adhere to surfaces and cause the wet paint to pull back from the contaminated spot, creating small circles called "fish eyes." It is very easy to use too much of the additive, which eventually causes the paint to delaminate or peel. Be careful.
Drying additives do just what the word implies - speed up drying time - but in addition they can improve hardness and gloss. In hot, humid conditions paints can lay on the surface and "fog" or "blush," in which case retarding additives can be used to slow down the drying process.
Flattening additives are solutions of minute particles that tone down the gloss of the surface. This is how various stages of gloss (semi, eggshell, flat, etc.) can be achieved.
Most of us are to some extent familiar with automotive paint, but it's worth a little background explanation of where the industry started and where it is today. Here's a timeline that shows the progression of automotive finishes.
1924 — Spray painting was introduced and the only available material was nitrocellulose lacquer.
1929 — Alkyd enamel was introduced. It was tougher than lacquer and needed no compounding. Most manufacturers used either lacquer or enamel from this point on, depending upon availability and factory conditions.
1940 — Chrysler and Ford changed over to enamels exclusively. Metal flakes in paints were introduced by the aluminum industry.
1956 — Acrylic lacquer was popularized due to its better sheen and durability. GM went exclusively to the material.
1960s — Paints utilized chromates, lead and other heavy metal chemicals that produced durable finishes. Ford called its enamels "porcelinized" paints.
1970 — The first aftermarket polyurethane enamel, Imron, was introduced by DuPont. It utilized a two-part chemical system that was extremely durable. DuPont experimented with waterborne solvents during this period.
1978 — High solids acrylic enamel becomes the paint choice of most manufacturers. These have the advantage of requiring fewer coats.
1980 — Base/clear coats are experimented with on Lincoln Versailles
1982 — Base/clear becomes industry standard.
1987 — Water-borne primers introduced. These paints are not actually dissolved in water, but use water as the medium to help transfer the paint from the can onto the car. They took a long time to perfect at the factory level, during which many durability problems were encountered.
1989 — Water-borne base coats introduced.
1990 — Clean Air Act sets strict limits on VOC's emitted during paint applications.
1997 — Anti-chip primers become industry standard.