Slow Dry
Slow dry, or softness as it's sometimes called, is when your paint ends up dry but soft to the touch and susceptible to retaining fingerprints or water spotting hours or days after it should be dry and hard. This can be caused by shooting your under or topcoats really heavy and wet, not allowing sufficient flash time between coats, adding too much or too little hardener to your paint materials, or using the wrong thinner/reducer for your temperature conditions. You can avoid slow dry by making sure it's at least 70 degrees where and when you spray, by using the correct hardeners for your materials and mixing them in the proper ratios, and by using the correct reducers/thinners for the temperatures encountered. Rectifying soft film can be accomplished by force drying if possible, or by removing soft film and re-prepping and spraying.
Shrinking
Also known as edge mapping or bulls-eyes, shrinking is when a repaired area's feather edging becomes visible shortly (within days) after a paint job is completed. Shrinking is primarily caused by shooting a topcoat before the undercoat has thoroughly dried. Or when you pile up multiple wet or heavy undercoats without sufficient flash time between 'em, or possibly even applying a finish over body filler that's not completely cured. This situation can be fixed by allowing the affected area to dry or cure fully and then sanding and refinishing as needed.
You can ofttimes prevent the situation by making sure your body filler is completely cured before priming, making sure you thin/reduce your undercoats per label directions, and by applying undercoats in lighter coats and letting them flash to avoid bridging sand scratches.
Solvent Popping
Solvent popping, or boiling as it's sometimes known, can be recognized by groups of small bubbles or crater-like openings in the paint surface. This condition is often caused by solvent getting trapped in the paint film when it skims over before all the solvent is allowed to evaporate. The solvents left under the paint film rise up ultimately "popping" through the surface leaving pinholes or craters. This is caused by the paint film skinning over because either too fast a reducer/thinner was used, or there was excessive air movement over and around the vehicle that dried the surface before the buried solvents had time to evaporate. This situation can be fixed by allowing the affected area to dry or cure fully and then sanding and refinishing as needed, or if it's severe, you may have to strip the affected area and then prime, seal, and recoat as needed.
Mottling
Mottling, also referred to as streaking, floating, or zebra striping, is a streaked, spotty, or striped appearance that shows up in a metallic or candy (transparent) finish. It's a tough one to show in an image, but in the case of metallics, it's when the flake flows unevenly in the wet topcoat resulting in pooling or flowing outward into a sort of ring, leaving some areas with less flake and some with more causing light and dark areas. Mottling with candies is more apt to look like zebra stripes rather than the flowing or pooling you see with metallics. Mottling can be caused by a bunch of factors, among them are an unbalanced spray pattern, tilting the spray gun so the fan is heavier either at the top or bottom of the pattern, over thinning or reducing the finish, applying a clearcoat before the basecoat has completely flashed, and improper overlap when making your passes. You can help prevent mottling by using the correct needle/nozzle/air cap combination on your gun, adjusting it for a proper spray pattern, keeping it perpendicular to the surface being sprayed, using the recommended thinning/reducing ratio when mixing your material, and allowing proper flash/dry times before clearcoating.
If you do end up with a mottling problem with a metallic color, you may be able to rectify it with a higher pressure mist coat while your previous coat is still wet, or allow the basecoat to flash and come back with a lower pressure mist coat. Just remember, if you allow a mottled panel to completely dry, you'll have to sand and refinish it from scratch.
Grit
Grit, sometimes referred to as seediness, is the dispersion of solid particles of different sizes embedded in the paint surface. This usually happens when your paint material isn't properly or completely stirred, or more commonly when you don't strain your paint or primer. You may also run into a grit problem when using old paint (like that can of color you've had stashed waiting for the "right" car to use it on) or by using material past its pot life (the amount of time before the catalyst really begins to kick in and starts to harden the material).
Repair options in the case of grit are the same as those from runs and sags-you can wipe the area with a solvent wetted rag and then clean and re-spray the area (seldom a first choice), or you can keep on going and wait till the paint fully cures and then sand and buff or sand and re-spray. Grit prevention can be achieved by mixing your materials thoroughly, straining all your under and topcoats, and mixing up only enough material to use within its specified pot life.
Blistering
Blistering, sometimes called pimples, are bubbles or swelled areas that show up in the paint surface weeks or months after a paint job. They're caused by moisture that's been trapped under the paint surface and is sometimes caused by spraying during really high humidity conditions. It can also be caused by contaminated air lines, failure to drain your compressor, or by painting over an unclean or contaminated surface. Blistering can sometimes be repaired by sanding the affected area and refinishing, but usually one has to actually strip the area to its bare substrate and start from scratch. This malady can be prevented by always draining your compressor and air lines, proper cleaning and prepping before painting, making sure everything is totally dry after wet sanding, and making sure you use the correct thinner/reducer for your spray conditions.
Discoloration
Discoloration, or bleed-through is a yellowish stain that appears in the topcoat over areas repaired using glazing putty or body filler. It's usually caused by too much, or in the opposite, too little hardener in the putty or filler. It can also be caused by incomplete mixing of either, or from priming before the filler/putty is fully cured. In minor cases it may disappear as the topcoat fully cures, but normally you'll have to sand, seal, and refinish the area. You can usually eliminate the bleed-through by making sure you don't use excessive amounts of crme hardener, by mixing fillers or putty completely, or by sealing any repair areas before painting.
Blushing
Blushing is a common problem when spraying in high-humidity conditions. It happens when the air and evaporating solvent from the spray gun lowers the temperature of the surface being painted to below the dew point (the temperature at which air becomes saturated and produces water). This condition causes condensation in or on the paint layer producing a smoky or milky looking cloud on the paint surface. Blushing can normally be corrected by adding a bit of retarder (a slow-evaporating solvent) to your paint mixture and then recoating, or by letting the finish cure and then compounding and buffing.
You can help prevent blushing by using a good-quality thinner/reducer that's correct for your conditions, adding the recommended amount of retarder when spraying in humid conditions, or by applying heat after application to help evaporate excess moisture.