Rattle can paint job!
There are two broad classes of paint, enamels and lacquers. Enamels undergo a chemical reaction (curing) as the solvent evaporates. In lacquers, the solvent evaporates without any curing. Now hobby paints are generally (oil-based) enamels, or aqueous (water-based) acrylic lacquers. The hobby enamels (e.g. Testors, Model Masters, Humbrol) clean ups with mineral turps, dries slowly but tends to harden beautifully to a glossy finish. The hobby acrylics (e.g. Tamiya) dries faster and clean up is easier.
The following describes methods using both enamels and acrylic based paints.
Rattle can paint job
The major advantage over paint brushing a project is that it can provide by far a much smoother paint job than brushing particularly on larger surfaces such as car bodies.
There are a host of different paints that are available these days including both model specific spray paints such as that available from Tamiya, Testors and Humbrol (both acrylics and enamels), general purpose indoor enamel spray paints available from hardware stores and art & craft stores and/or automotive paints available from auto stores.
My preference is automotive paints as there are generally a lager range of colours available, I find the paints generally dry a lot faster and because the paint has a harder surface once dry, I find it easier to cut and polish. However having said that I do also use other paints such as the model specific paints and generally find that the steps I use to paint does not vary much between the different types of paint.
Most of the painting projects I do are re-spray jobs of slotcars so the first step in the painting process of the model is to strip down the car to all its individual parts. once I have all the parts off the body I then start the exercise of removing the existing paint. This is important especially if the car has a lot of numbering and sponsorship as it will have raised surfaces from one painted surface to the next and those contours could come through the paint job you do!
I am yet to find that all in one solution to stripping paint from a car and so the process will differ greatly from one manufacturer to another, for example: I find both Scalextric and Fly a lot easier to remove paint from than say SCX and unfortunately to date the only way I have successfully removed the exiting paint work from the later is by elbow grease and sanding it all back. Other brands can have various forms of paint stripper, caustic soda or other solvents applied to remove the paint, but always be vigilant that you don't also dissolve the plastic body under the paint with some of these chemicals.
Once this step is completed I give it a good wash in some mild soapy water and get ready for the next step.
For some of my projects I actually want to change the visual look of the model, in some instances its a simply a matter of puttying vents or other holes left from rear wings and the like that I don't want or adding flares to guards or in extreme cases chopping roofs are other major mods to the body. Once this work is complete, I prepare the body by giving another wash in mild soapy water, making sure its completely dry and giving a light coat of primer. [1, 2, 3 & 4]
The primer I almost exclusively use is either the grey or white automotive primer. By doing this step you will see very quickly any imperfection that you could not see previously. Once I have sanded any imperfections and tweaked any of the work that I had done on the body I get it ready for another coat of primer this includes giving it a sand with a wet and dry 1200 or finer sand paper and then washing it again in soapy water prior to giving it its final primer coat.
Once dry you can do a final check of the body for any imperfections and let it gas out ready for colour coat. Generally I leave it for at least a day, but if you have the patience a week would be fantastic! 
Once your primer coat has fully cured you can give it another final sanding, again with a very fine wet and dry and wash it ready for its colour coat. 
Start the colour coats with a really fine misting coat first, I normally do this by holding the work a little further away from the spray can than otherwise recommended and passing over the job a little quicker. As it is a mist coat you can generally apply a second and subsequent coats within 15 to 30 minutes but try not to leave it too long, say an hour or so between coats, otherwise you will need to set the project aside for 24 hours or run the risk of the paint not adhering or worse bubbling. 2nd and 3rd coats are generally applied a little thicker but not by much. Again this can be achieved by holding your work a little further away from the spray can than as directed and slowly building the paint up. 
Once you have done a few mist coats and you are happy with the coverage it is time to do a final wet coat! This time when you spray the surface spray at the recommended distance from the object or a smidge closer and go slightly slower over the pass. What you are trying to achieve with this coat is a wet shiny look (thus wet coat). The trick being however, is not to do too heavy a coat as you run the risk of the paint running. 
Again I let the model sit to gas out and in most cases will leave it for at least 3 days especially if I plan to apply decals to it.
If I am not applying any decals I give the model another fine sanding with wet and dry and apply a top clear coat again with an automotive clear if the colour coat is an automotive paint, I set it aside again to gas out and then do a final light sanding (if required) and cut and polish.
If I am applying decals to the model I apply the decals and set aside for a day and then apply a top clear coat but this time I use an acrylic based clear called Crystal Clear. I use this as I generally make my own decals which are printed from a bubble jet and/or from an Alps printer and Crystal Clear is what is used to seal the decals prior to use. Again once this process is completed I do a final cut and polish. If I were using a commercially available decal I would use the manufactures recommended sealer/top coat for the job.
- Different types of paint with different purposes are often applied to the same model, in multiple layers. Some of the typical ones are primer, colour coat and clear coat.
- The primer prepares the surface for the following coats of paint and tends to bond aggressively with the plastic. The primer is also designed for the colour coat to bond well to it, reducing the tendency to peel or lift.
- Colour coats are built up in layers, starting with light mist coats and eventually building to a wet coat.
- Clear coats are used for a number of reasons, including: protecting the painted surface, protecting the decals and providing a depth and brilliance to a paint job.
Paint compatibility matrix
|Acrylic over coat||Enamel over coat||Lacquer over coat|
Polishing and Waxing
After clear coat, the paint job should look good but you can go that one step further and make it look even better with a bit of extra work in polishing. Polish and wax are both very fine abrasives. You can use model car specific compounds such as Tamiya Fine, Medium and Course polishing compounds which is what I mainly use. But I have also had success with polishes designed for our models 1:1 counterparts as well and these can work just as well and also are more economical as you may already have a stock of various different polishing compounds and waxes in the garage. Just be careful with some enamels and using polishes that are coloured as the pigments in the polish could effect lighter coloured paints.
If the clear coat is not perfect, you can lightly wet-sand it with 1800 grit then 2400 grit or finer. Find a lint-free cloth and apply a small amount of polishing compound. Spread the polish onto the area gently, then start rubbing in a circular motion, stopping to check your progress frequently. You'll notice that the surface gets more and more reflective.
Avoid putting too much pressure on corners and edges. At these points the top coat can be removed without too much pressure. Check the surface of your polishing pad to make sure color coat is not being removed. After polishing the entire
piece you are working on give it a wash with soap and water and dry.