An overview of automotive paint systems

Courtesy Rohan Borrell

One of the first things we notice about a car is the colour.  When we are buying a car the condition of the paint work gets a lot of attention.  There is a good reason for this because paint work is expensive to repair.  Poor paint work might also indicate poor repairs which can be hiding serious structural damage.  So when it comes to paint what are we talking about?

Automotive paints can be split into three general categories;-
Lacquers
Enamels
Urethanes, polyurethane's & epoxy paints which are usually 2 part.

Acrylic is another term that is often used when discussing paint types.  Acrylic can be taken to mean "plastic" as opposed to paint made from naturally occurring substances (e.g. shellac lacquer).

Lacquers

Lacquers rely on solvent evaporation and require thinner layers of paint to be built up with many coats.  Lacquers dry very quickly minimising the time for dust to land in the paint and speeding up application.  Lacquers are still used by some custom car builders because of the ability to polish lacquer to a very high finish.  It is also easy to use and clean up.  Small areas can be resprayed easily.  Since lacquer over spray dries very quickly it lands as dust and can be wiped off adjoining surfaces.  This is not the case with enamels or 2 part paints.

The main drawback of lacquers is the high content of solvents (Volatile Organic Content).  This can cause all sorts of solvent attack problems with the paint being painted over.  There are sealers than can reduce this problem.  There is also the problem of blushing in humid weather due to the evaporation of the solvent causing water in the air to condense on the paint.  Lacquers do not have the solid content of the other paints and therefore it takes many more layers to get the required paint thickness.  The evaporation of solvents is also an environmental issue hence are not often used commercially.

Lacquers were popular in with auto manufacturers from the 1930's - 1950's.

Enamels

Enamels rely on a smaller amount of solvent for an initial drying but then the enamel undergoes polymerisation to completely dry over several days.  Enamels need to be stored in air tight containers otherwise contact with oxygen will cause polymerisation.

When spraying enamels the paint is wet longer than lacquers so more susceptible to having dirt and bugs land in the paint.  Enamels are touch dry after about 1 hour but not fully hardened for several days.  If enamels are sprayed outside of a spray booth the over spray will land on adjoining items and cannot be easily removed.

Enamels have a higher solid content so less coats are required to achieve the required film thickness.

Enamels were popular with auto manufacturers from the 1950's to the 1980's.  To complicate things there are many types of enamels with acrylic enamels becoming popular with auto manufacturers during the 1960's.

Metallic paints used in the 60's were often an acrylic enamel.  These are sometimes called mono metallic since there was no clear coat over the metallic.  These metallic often had very poor durability in the sun and weather.  It is also difficult to get a uniform metallic effect with these paints.

2 part paints

Two parts paints commonly known as 2 pack or 2K are usually urethane.  These systems consist of part A (paint or resin) and part B (hardener or catalyst).  These are mixed in a specified ratio which causes the mixture to undergo an irreversible reaction and harden.  The catalyst (hardener) contains isocynate.  This is a serious health hazard and can be absorbed in to the body via skin contact and inhalation.  Caution needs to be used when spraying these paints.  They should only be applied in an approved spray booth with appropriate protection.

This system has become standard for new cars and repairs on modern vehicles.  A Base Coat/Clear Coat (COB) system is always used with metallic and sometimes with plain colours.  The finish with the 2K system is extremely durable and inert to many chemicals which would attack enamels and lacquers.  For example 2K will resist attack by brake fluid, thinner and most acids.  2K paints are also the most expensive with 1 litre of high quality 2 K paint being around $100 plus hardener which is another $80.  The finish and durability of these paints is exceptional.

Other systems

There are also "Rust Repair" paint systems that are single part urethane.  These rely on oxygen and moisture absorption to cure.  Examples of these are the POR (Paint Over Rust) system or KBS (similar product).  Again these paints are very stable once cured and do not need an etch primer.  However for a good finish these paints need to be coated over as they often dry with imperfections and discolor on exposure to UV.

There are many variations of the above systems and the manufacturers instructions always need to be read carefully to fully understand the limitations of the product.  Each manufacturer's product will be slightly different and needs to be treated as such.

Which paint is right for you?

Of course there is no easy answer to this.  For repairs to a section of an older vehicle lacquer would be a good option particularly if you want to do it yourself.  Most people with practical skills will be able to achieve a good result with some practice.

Lacquer can also be good for restoration of parts that are not exposed to the weather and do not require a high gloss finish.  Lacquers have to be polished to give a good finish.  If the part is unable to be buffed and polished you will need to be happy with a lower gloss level.

Enamels are good for items that need a higher gloss level without buffing.  Examples where enamels would be used are engine and driveline parts as well as other under body parts.

This panel has coloured primer surfacer to approximately match the top coat colour. 

Single pack black urethane has been used on the inside of the panel

To use 2K you need an approved spray booth and protection.
Most resprays will be done with 2K paint.  2K paints allow a base coat/clear coat system to be used with metallic and plain colours.  Some of the restoration purists frown on using a base coat/clear coat system on restoration of classic cars.  The thinking is that the effect and finish of base coat/clear coat is much higher than the original paint taking away from the original look.  Obviously this is a personal preference.  Most modern repair shops will only use 2K systems.