Understanding Today's Automotive Paint

There has been a steady evolvement of paint materials, here is a basic guide:

Paint, basically, is composed of three items: pigment (the colour), resin (the medium to suspend the pigment and protect it) and solvent (the medium to keep it liquid to aid in transferring resin to the surface).

For a finish to last (or “weather”), a minimum of 2-2.5 mils is required on the surface and the “as-sprayed solids” determine this and often the number of coats to be applied.

Enamel: A glossy oil based paint, enamel was brush-painted on cars then sprayed on vehicles since the 1930’s. Enamels feature a very shiny appearance and are easy to apply. They usually need a long cure period - at least 60 days to be fully cured as most enamels do not use a chemical catalyst (hardener). They dry quickly on the top surface but will remain “wet” underneath. Due to this, they cannot be recoated without reacting until cured and they lack chemical resistance. Generally, due to of the high amount of solvent and poor cure times required, they are not used in refinishing much anymore. Enamels are prone to fading and loosing gloss after constant sun exposure. Solids (pigment and resin left after spraying and solvent evaporates) are around 10-20% as sprayed.

Acrylic Enamel: A hybrid of enamel, adding acrylic resins significantly improved the performance, including chemical and fade resistance and was used widely by the refinish trade as well as manufacturers from the mid 60’s to early 80’s. While better than synthetic enamel, it still lacked quick full cure and was susceptible to chipping, as it remains brittle when cured. Slightly higher in solids than enamel, running approximately 15-25% as sprayed.

Nitrocellulose Lacquer: Used in early refinishing, this material was very quick to dry and very easy to use, was prone to fading and chipping, and required polishing to achieve high gloss. While more labor intensive than enamel (due to the compounding), the final finish was very smooth and allowed (and required) the use of more coats due to the quick flash times, enabling the first multiple coat custom colors and show finishes. Preparation for a lacquer finish was fussier than enamel. Very low in solids at 5-15%, nitro finishes need lots of coats.

Acrylic Lacquer: By using an acrylic resin (TPA or thermal plastic acrylic), lacquers would not fade as before, and offered a medium for the first custom finishes that would last. Used by GM from the 60’s up to the late 80’s, it offered an easy to use finish but was susceptible to chipping and required polishing for high gloss. To save production cost, GM used a non-sanding “dispersion primer”, which unfortunately, caused cracking in their finishes with time, a result of the top coat (acrylic lacquer) curing very hard (drying out) and their undercoat remaining uncured and moving. Lacquer finishes need more care to retain gloss and can bleach from constant sun. Combined with labor and high solvents, lacquer is rarely used today. Solids on acrylic lacquer are slightly better than nitro at 10 to 20%, again many coats are needed for correct film build.

Acrylic Urethane Enamel: First introduced by the Germans in the 70’s, these finishes offered a real wet look with superior chemical resistance and quick curing. They are primarily used for solid colors, as they tend to loose some gloss when metallic is added. Most automotive clear coats today use an acrylic urethane resin. Urethanes are less brittle than regular enamels and have better gloss retention over time. These colors are referred to as “single stage” or 2K (as they require hardener, hence two component – spelled with a “k” in German). Solids in acrylic urethane vary from 25 to 50% requiring fewer coats to achieve correct film build.

Base / Clear: Again first introduced in Europe, these finishes afforded the ease of application of a lacquer for metallic (quick setup) and the advantages of a urethane clear for lasting shine, quick curing, and superior chemical resistance. By applying a high solid base of color that covers and dries quickly followed by a tough urethane clear coat for shine; fast, easy to apply glamorous finishes were now available. Most vehicles at the factory are painted this way today and base/clear repairs have been used in refinishing widely since the early 90’s. They also afforded, for the first time, the easy use of mica (pearlescent) pigments to be used at the factory and in refinish which have replaced most metalics and allowed many new colors being available. Most basecoats are of urethane or polyester base although water borne resins are used in some of the car plants while the clear coats remain acrylic urethane. Solids on basecoats vary from 25 to 60%, while clears vary from 25 to 50% as sprayed.

Poly-Urethane Enamel: By adding polyester to acrylic resins, a hybrid finish was created, primarily for aircraft and big trucks where a super durable finish was required that would last and last. Developed in the U.S., polyurethane finishes and clears appear primarily from American manufacturers. The solids vary from 25 to 60% as sprayed.

VOC compliance: In an effort to cut down on solvent emissions (VOC or Volatile Organic Compounds), low VOC paints are now widely used in the refinish industry and by law in the U.S. Low VOC (or V.O.C. compliant) products feature more solids (pigment and resin) with less solvent and generally require less coats to achieve proper film builds. In order to work they generally use slower solvents and tend to be more susceptible to airborne contaminants. As well, they are harder to apply smoothly unless you have very clean, temperature controlled conditions.

HVLP spray equipment: Also to cut down on VOCs, HVLP (High Volume / Low Pressure) guns apply paint at approximately 10 PSI out the cap (compared to 40-55 lbs with a conventional gun) and are now widely used. They generally require much more air (CFM) to work properly and sometimes require different application techniques to get a smooth finish. HVLPs cut down on overspray and thus material usage due to the lower spray pressures.

Thermal Plastic and Thermal Set finishes: Lacquers and uncured enamels are referred to as thermal plastic finishes which can be re-penetrated and softened by solvents. Thermal set finishes generally are chemically cured from a catalyst (hardener) and cannot be penetrated or softened by solvent.

Got all that? (editor's note: remember that John will answer questions. His email is above and on the main page)

John