I've never used primer through my airbrush. I've always used spray cans for that. Mostly Tamiya.
As for amount of color paint, usually one bottle should cover a 1/25th scale car.
When painting, I usually use several very light coats. I pause long enough between coats to let the paint flash off. When I get complete coverage, I go for a heavier coat.
Years ago, on another forum which is now gone, they had a tutorial on painting which I saved. Here it is......
The Basics of Spray Painting
I am not going to go into every detail here, but I will try to cover the basic principles and hopefully give you a good place to start from.
Most of the info here comes from 3 places, the Painting forum of Brian's Model Cars, the Internet as a whole where I have spent many hours seeking out better technique, and lastly, my own experience.
You can't lay down a good paint job unless the surface you are painting has been properly prepared. Things that look like a minor blemish on an unpainted surface, once covered with a few coats of paint, can become a major eyesore.
To get a good paint job, the layers need to be as thin as possible; this will prevent small details being hidden.
Spending time preparing the surface, making sure it is as smooth and as blemish free as possible will only help you to achieve a better paint finish.
Priming
After preparing the surface, it is recommended to prime the surface, this helps the paint to stick to the surface properly.
For the paint to bind properly to the surface of the model, it must be free from oils, dust and other contaminants, to ensure this it is important to wash the model thoroughly with a mild detergent and warm water before painting. Be careful with the temperature of the water as hot water can cause the plastic to warp.
Once the model is clean and dry, spray a thin coat of primer, this does two things, it gives the color coat a surface to adhere to, and helps to highlight any defects in the finish that may need correcting. Problems are generally easier to see with gray primer than they are with white primer. If you find any problems, fix them and respray with primer.
If the model is going to be painted with a light color, then a thin coat of white primer over the gray primer is recommended.
After priming, the model should be set aside to dry for at least 24 hours.
Painting the Color Coats
This comes down to more than just laying down a layer of spray paint, getting a good color coat starts right at the point of preparing the paint before spraying.
Preparing the paint involves mixing it for use in the airbrush, or shaking the rattle can for a sufficient amount of time and warming the paint in the rattle can.
When it comes to preparing the paint for the airbrush, it usually involves mixing the paint with the relevant solvent (acrylic thinners in the case of acrylics and so on), to getting the paint to the correct consistency for spraying. The general rule of thumb for airbrushing is to get the paint to about the consistency of milk. This may sound strange, but to get an idea of what is meant, place some milk in a glass and swirl it around a little, this will give you an idea of what to aim for when thinning the paint.
When it comes to preparing the rattle can, you want to thoroughly shake the can to make sure the solvents and pigments in the can are completely mixed, normally shaking for 1-2 minutes should be sufficient. Another invaluable tip I have learnt, and which I do religiously now, is warm the spray can. Do this by simply placing the can in some warm water (it must not be hotter than you can handle with your hand), by heating the can it helps the paint flow better when spraying since the warm paint has an increased viscosity, I personally believe it also helps maintain a good constant pressure out of the can too. Remember another thing, by heating the can, the solvents will evaporate faster, you will understand why I am mentioning this next.
When the trigger on the airbrush or nozzle on the rattle can is depressed, the paint is atomized; this means there are tiny droplets being shot through the air towards the model. Upon landing on the model they spread out and join together to form a uniform surface. The smaller the droplets the thinner and more even the coat of paint. At the same time the droplets are spreading out, the solvent are evaporating, which stops the paint flowing as it dries, if the solvent evaporates too soon, it doesn't have time to level properly and you will end up with orange peel effect, at worst it can evaporate before the paint reaches the surface and you up with a surface that looks like flat paint.
If you are constantly getting orange peel, it is likely that it is caused by one of two things, one is that the paint is too thick in the case of airbrushes, the other is that you are spraying from too far away. Trial and error will help you find the best distance to spray from and how thick the paint should be for your application.
Now to start spraying, this is where patience becomes a virtue, many spray jobs have been ruined by people rushing through it and then trying to fix the mistakes after.
When spraying, never start or stop the spray over the surface of the model, start the spray one side, spray over the model in a straight line keeping an equal distance from the model, and after you have passed the end of the model with the spray. This will prevent any drips that could possibly build up on the nozzle from landing on the model and potentially ruining a good paint job.
A basic run down of possible problems and how to solve them is as follows:
1. Orange Peel:
Cause: Paint is drying before it has a chance to level
Fix: Sand to level the surface of the paint once the paint is thoroughly cured
Solution: Heat the paint (spray cans), Move closer to the subject
Tip: If you move closer or heat the paint, you will be increasing the amount of paint you are laying down so you will need to move over the subject faster
2. Flat or Dull gloss:
Cause: Paint is drying too much before it hits the surface of the subject
Fix: Light wet sand and repaint
Solution: Move closer to the subject
Tip: Same as for Orange Peel
3. Sags or Runs
Cause: Paint is too thick or drying too slowly
Fix: Sand with a sanding stick to level the area and repaint the subject
Solutions: Heat the paint in the case of rattle cans, move slightly farther from the subject, move across the surface faster
Tip: It is easier to add paint than remove, this is often caused from applying too much paint, apply thin layers rather than try to cover everything with one layer.
4. Fisheyes (Dimples):
Cause: Surface contamination
Wet sand the are down to primer
Solution: Always carefully wash surfaces before painting
Tips: Painting is about clean flat surfaces; by thoroughly washing you can prevent things like this.
5. Blobs of Paint
Dirty nozzles or tips and old lumpy paint
Solution: Sand the surface smooth with a sanding stick and repaint the subject.
Solution: Always clean paint cans or airbrushes thoroughly after use per the manufacturer's instructions.
Tips: Thoroughly mix old paints and clean spray nozzles/airbrush tips, even if you will be spraying with them again in a short while.
Allowing paint time to dry between coats is important, if the solvents don't have enough time to evaporate between coats, the paint will remain soft longer, at worst it may even crack over time because of uneven drying.
When it comes to paints and drying times, it is totally dependant on the paint type, enamels will take at least 7 days to be completely dry, lacquers will take at least 2 days and acrylics at least 4 days. Between mist coats the minimum drying times are; minimum 4 hours for enamels, minimum 20 minutes for lacquers and minimum 1 hour for acrylics. Remember that longer will always be better.
It is mostly recommended to lay down several 'mist' coats before applying a 'wet' coat to allow even build up of paint. Using a primer can help eliminate the need for too many mist coats.
Once a good coverage is acquired with mist coats, it is time to lay down the final coats otherwise called the wet coats, this is when the paint creates the shine you are looking for. The basic rule is to spray until the paint looks about ready to run, this is really a practiced technique in many ways, I know it sounds risky, and I am sure the first couple of times you may end up with a run or two, but practice and time will help.
Never expect paint to form a complete cover on the first coat, this is the way to run into any number of problems described, rather build up the paint layers gradually and evenly.
Any defects you notice should be repaired between coats, you should never hope the next layer of paint would cover something up; paint hides nothing once it is dry. The most recommended method to repair paint blemishes is careful wet sanding, before sanding though, you should always allow the paint to dry completely for at least 24 hours.
Painting with either an airbrush or rattle can is a technique that is developed by the individual, but being well armed with information does not hurt.
My best tip when it comes to laying down a good paint job on a model is practice and patience.