How are automotive paint differ from building paints?
They stick to metal, other paints are made to stick to plastic or fabric or fiberglass, etc. Different chemical breakdown and compatibility
emulsion paint could basically be thinned with water. Gloss etc must be thinned with spirits like white spirits or turpentine. you may study the guidelines on the tin. There are paint thinners attainable at any DIY save and Tesco/Saisbury's/ Morrisons/Wilkinsons etc. Even the markets!!
I am in the process of painting my house and I am painting and restoring a fiberglass car. Most exterior house paint is latex and water based. It is a one part process, you dump it in the bucket and paint it on the building. Automotive paint is different. House paint needs to be able to give a little. not so much on a car. adhesion is an issue too. modern automotive paint is applied in two parts. base color coat which is the color mixed with a reducer Clear coat which is mixed with a hardener. the clear is what adds the depth and protection to your paint job. by building up the clear when you paint a car, you are able to color sand or sand it down with very fine sand paper. this smooth clear is what makes it have that mirror shine. building paints wont shine or buff out like that.
Paints for a house are very simple. There's a pigment, dissolved in some solvent. You lay out the paint, and the solvent evaporates, leaving the pigment behind. Auto paint is much more durable, but also harder to apply and very toxic. Modern auto paints are two part 'catalyzed' products. Paint is mixed with an activator, and sometimes a reducer, and then applied. The activator makes the paint molecules 'cross link'. The paint film is bound on a molecular level, making for better adhesion and strength. If you want to know a ton more, look at the link I give. BarrkK owns Southern Polyurethanes, a paint company that is well known in the custom/restoration circles. If you read that forum and read what he says, you will eventually know everything there is to know about paint. Also check the wikipedia articles on polymers and cross-linking
