Dust Or Dirt
Dust or dirt in the paint is by far the most common of all paint headaches, and actually for those of us (read that most of us) that don't have access to a 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 truck's surface is clean, especially around the jambs, bed, and underhood. Don't forget to tack off your masking paper and tape masked areas as well. And you should use a fresh 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. Unfortunately, a clean, professional booth is the only real solution to eliminating dust. For the guy at home the best way to get rid of dust in the paint is to follow up with a good colorsand and rub.
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 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). Another thing that helps to prevent feather edge lifting is to always final sand a repair area with 400-grit of 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.
Checking, sometimes referred to as crow's feet, are cracks of various lengths and widths that show up in a topcoat. This is one problem that has a number of possible causes. The most common causes are excessive film thickness, too short of a flash time between coats, force-drying the undercoat (like using the blowgun to dry primer), and sometimes by using too much hardener or catalyst in the primer or paint. The only way to fix checking is to strip all crazed and cracked paint film and do the entire job over.
Mottling, also referred to as streaking, floating, or watermelon stripes, 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 watermelon stripes rather than the flowing or pooling seen 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 passes with the gun. You can help prevent mottling by using the correct needle/nozzle/air cap combination on your gun, adjusting your gun for a proper spray pattern, keeping the gun perpendicular to the surface being sprayed, using the recommended thinning/reducing ratio when mixing your material, and allowing proper flash/dry times before clearcoat. 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.
Orange peel is another of the more common paint problems 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 1,200-grit or finer paper and then buffing, or sanding and respraying 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.