Types of Automotive Paint
There are three different automotive paint types or systems you are likely to come across.
This is the original automotive production-line paint system, going back beyond the old Model-T Fords. The paint is thinned down by large amounts of cellulose thinner, which evaporates after the paint is sprayed onto the bodywork. Up to 50% of the volume of the paint can evaporate away as it dries. It's easy to apply with basic breathing apparatus and with relatively cheap spraying equipment. It's a system which isn't designed to produce a final finish out of the spray gun. Rather, once applied it needs to be polished to achieve a high gloss shine. Unlike more modern paint systems it can be applied by brush, although the best effect is achieved by spraying. Solvent-based coatings are now prohibited in many countries for enviromental reasons.
Two-Pack or Isocyanate paint
In the 1970s, manufacturers almost all switched to two-pack paint. This paint is made up of two resins which you mix together. The resins react with each other chemically and start to harden. The paint is extremely toxic and can affect breathing if inhaled with potentially fatal results, so special breathing aparatus must be worn while it's being applied. Two-pack paint is very hard-wearing and chemically resistant, and produces a finish straight from the spray gun, so application times are shorter. Because of the toxicity, this paint tends to only be used by professionals and more experienced amateurs.
This is a relatively modern paint system, but is very commonly used by car manufacturers and repairers. In fact, all European manufacturers use water-based paint on new cars by law. It's better for the environment due to a reduction in the amount of volatile organic compounds. The colour coat is water-based but still needs to be protected by a lacquer coating, similar to two-pack paint. This is harder to use for DIY painters due to the need for two different paint types.