Orange Peel
Orange peel is another of the more common paint problems we run into, and its name is pretty self-explanatory. It's an uneven paint film that has a texture that, well, looks like an orange peel. This predicament is more often than not caused by under-thinning/reducing the paint, spraying at too low a pressure, or a combination of both. Other causes may well be too fast a thinner or reducer, piling on too many or too heavy coats, or improper spray gun adjustment. Depending on severity orange peel can be repaired by compounding and polishing, or wet sanding with 1200-grit or finer paper and then buffing, or sanding and re-spraying the surface.
You can usually prevent orange peel by thinning/reducing your paint according to label instructions (are we beginning to see a pattern here, folks?), using the correct speed of thinner/reducer for the ambient temperature, using the correct air pressure, and avoiding really heavy coats.
Edge Mapping
Edge mapping, also known as feather edge lifting or edge ringing, is caused by the solvent from a fresh topcoat penetrating a sensitive area of an undercoat (most commonly the featheredges of a repaired area). You'll recognize it as a wrinkled area outlining a repaired area. When painting a vehicle with repair areas (meaning almost everything those of us who can't afford a new steel aftermarket body will end up painting), it's always a good idea to use a two-component primer surfacer, water-base primer surfacer, or an appropriate sealer that'll create a barrier between the repair and the fresh topcoat.
If you do encounter edge mapping, you'll have to sand, smooth, or remove the affected area and seal it with a good barrier coat of some kind (your paint supplier will be able to recommend one for ya). Another thing that helps to prevent featheredge lifting is to always final sand a repair area with 400-grit or finer paper-the finer the final sanding, the shallower the sand scratches will be, and there'll be less area for the solvent to attack.
Air Trapping
Air trapping causes small craters that are similar in appearance to fisheyes. These are caused by tiny air bubbles trapped in the fresh paint film that rise to the surface and "pop," causing small, crater-like depressions. These "craters" are usually caused by under atomization of the paint due to either too low of an air pressure setting, improper spray gun adjustment, spraying with your gun too close to the surface, or moving your gun too slowly across a panel. Depending on its severity, cratering can be repaired by either sanding with 1200-grit or finer paper and then polishing to restore gloss, or by sanding and re-spraying the area. You can avoid this problem by maintaining correct spray gun speed and distance, making sure you've got the right cap/nozzle/needle setup for the type of product you're spraying, and making sure you're using the correct recommended air pressure.
Sand Scratches
Sand scratches show up as lines or marks in the paint film that mirror the marks in the surface being painted. They may also show up as streaks in the topcoat that magnify marks in the undercoat or substrate layer. These are caused by improper or incomplete final sanding of bodywork or primer coats (using too coarse of a paper), trying to cover scratches by filling 'em with primer, or in some cases by sanding single stage or basecoat finishes before clearing them. You can fix 'em by letting the finish cure and then carefully re-sanding the area with an ultra-fine paper and then refinishing it.
You can avoid sand scratches by graduating your sanding from coarse to fine papers, and not sanding basecoat colors before clearcoat (though if you do have to sand the basecoat for some reason make sure you apply additional basecoat color before clearing). Use 1200-grit or finer paper for color sanding.
Crazing
Crazing and cracking is a condition in which cracks (or lines) of different lengths and direction form in the finish. This is caused by excessive film thickness of either the topcoat or undercoat. It can also be caused by shooting over a previously crazed surface, using too much hardener in either your primer or paint, not thoroughly mixing your spray materials, or using off-brand or another brand of reducer or hardener in your mixture. The only way to correctly fix this paint problem is to strip the area and refinish it. You can usually prevent crazing by always following the material manufacturer's label instructions, by always using a manufacturer's complete line of products (no intermixing of different brands), and making sure you mix your coatings completely before spraying.
Peeling
Peeling or delamination is a loss of adhesion between a paint film and the substrate (the material being painted), causing sections of paint to separate from the surface. Though all paint problems are aggravating, there's nothing worse than leaving the driveway with a shiny new paint job and arriving at the donut shop in a bare metal car. Peeling is most commonly caused by poor surface preparation, usually insufficient sanding or cleaning. But there are other causes, too, like omitting or using the wrong primer for your substrate (the surface being painted), exceeding the paint product's recommended recoat time, or in the case of clearcoat colors spraying the colorcoat too dry, using an incompatible clearcoat, or incorrect colorcoat reduction.
You can prevent peeling by, again, reading the damn instructions for the products you're using, properly cleaning and sanding your substrate, using the correct undercoats (primers) for your substrate, and making sure you topcoat is within the recommended flash times for the material you're using.
Pinholing
Pinholing occurs in and over body filler or putty when air bubbles are trapped inside the fillers during mixing. These bubbles are then exposed during sanding creating small holes or craters in the surface. Sometimes the air or gas trapped in these pinholes will affect the topcoat by rising to the surface. Filler-caused pinholing problems are often created during the mixing of the hardener when you whip the filler/hardener mixture in a rapid circular motion. Keep in mind, you're not making a cake; filler should be mixed by consistently folding the mixture over itself until the hardener is fully dispersed. Too much hardener will also cause pinholing to become more likely. One last possible cause is excessive filler thickness. Globbing on huge dollops of filler (rather than trying to repair a dent) produces a lot of heat as the filler/hardener mixture catalyzes, and could cause gas formation and pinholing.
If you do end up with a pinholing (and you notice it before painting) situation you can apply a thin coating of spot putty or polyester glazing putty and sand it smooth, filling the pinholes and hopefully correcting the problem, though this route is more of a Band-Aid.
Dust 'N' Dirt
Crap in your paint is by far the most common of all paint headaches, and actually for those of us (read that most of us) who don't have access to a big-buck spray booth, one that we'll never completely rectify. With that said, all we can do is work to reduce the amount of dust and dirt as best we can. This can be accomplished by making sure that your spray environment (garage in most cases) is as clean as possible. Wet down the floor before you spray, and if possible between coats. Make sure the vehicle's surface is clean (I'm talking about all around the windows, door, hood, and trunk jambs). Don't forget to tack off your masking paper and tape masked areas as well. And you should use a fresh new tack rag as often as possible. Another thing a lot of people don't think about is static electricity-it really does come into play in this situation by actually attracting dust to the vehicle like a magnet (especially after all the compressed air blowing and hand wiping done during your final prep). Running a ground strap from the chassis of your vehicle to a good ground in the garage will help out more than you could imagine. I've tried it, continue to do it, and think it really does help.
Personally I've given up hope for a completely dust-free paint job in my home two-car garage-I've sometimes been able to bury very minor dust spots using another coat of color, but in most major dust cases the surface will have to be sanded with 1200-grit or finer paper and either buffed or re-sprayed. Unfortunately, a clean, professional booth is the only real solution to this one.