If you've ever visited an auto-parts store, you've probably spotted a whole section of automotive touch-up paint in spray cans. They're car paints, and you're building a car model, so everything should be fine, right? Well, yes and no. These paints can be used on plastic models, but only with preparation.
Spray paints in hobby shops are specially formulated for plastic models - they have a "cool" chemical composition that's harmless to delicate plastic parts. Paints in auto-parts and hardware stores, however, are quite the opposite. Most of these fast-drying paints contain "hot" solvents that can damage plastic. To use one of these hot paints on plastic, you'll need a barrier of primer between the paint and the plastic.
Where can you find these protective primers? Usually in the same rack as the paint. Each brand of paint usually includes two or three different primers. Read the labels carefully to find one that's made for use over plastics. A thorough coat of the protective primer will guard your carefully prepared bodywork and allow you to apply the color coat of your choice.
As always, there are precautions to consider. If you're not sure whether the primer-and-paint combination will be safe to apply over plastic, your best bet is to test it on an unwanted model or a piece of sheet styrene before you start shooting your latest masterpiece. Also, make sure you have plenty of ventilation, and always wear a good, two-stage respirator mask for protection.
A car body is tough to hold while spray painting, but a wire coat hanger can be bent to shape for a painting stand. Attach the stand to the inside of the model with masking tape. Make sure the model is attached securely - you don't want your just-painted masterpiece falling onto the floor!
Automotive spray paints are designed for use on full-size cars, and their nozzles produce a "full-size" spray pattern that's much larger than regular hobby-shop spray paints. If you've never used paint with a nozzle like this, practice on an unwanted model until you get the hang of things.
Apply the primer coat in light layers until it's completely covered. When the first coats are dry, remove the model from the stand and apply primer to the areas that were covered by the masking tape. This will protect the entire body from potential damage, not just the outside. After the primer is dry, sand it smooth with fine-grit sandpaper. (If you accidentally sand through the primer, apply another coat to the area and sand it smooth.) When you're finished, wash the model thoroughly with soap and water to remove any sanding residue.
Reattach the model to the coat hanger painting stand, again using plenty of masking tape. When it's secure, you're ready to paint. Apply a "mist coat." That's a light overall layer that gives final "color coats" something to stick to. Allow the mist coat to cure for 10 minutes. Now apply the heavier, glossy color coats. Each spray pass should cover the model evenly, producing a smooth, glossy finish. If you don't apply enough paint, the surface will be rough; too much and the paint will sag and run. Practice will perfect your technique.