Die back, also known as dulling or hazing, is the dulling of a finishes gloss or shine as it dries or ages. This one is pretty common and has quite a few different causes. I’ve found that you’ll be more susceptible to die back if you don’t allow adequate drying or curing time of your undercoat, or if you close up a freshly painted vehicle in a booth or garage with no air circulation. The latter cause I believe happens because in a sealed environment the evaporating solvents from the new finish hang around in the air and react with the still drying paint surface causing it to dull out. Other possible causes are to short a flash time between coats, using cheap off brand thinners or reducers, and sometimes an excessively heavy and wet final coat. You can repair die back by letting the finish dry thoroughly and then cutting and buffing it or you can sand and refinish. You can also help to avoid the problem in the first place by applying your topcoats according to the products directions, allowing sufficient flash times between coats, using the correct and/or recommended thinners or reducers, and making sure you’ve got good air flow around the vehicle as soon as its tack free.
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 metallic’s 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 candy’s is more apt to look like zebra stripes rather than the flowing or pooling you see with metallic’s. 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, adjust your gun for a proper spray pattern, keep the gun perpendicular to the surface being sprayed, use the recommended thinning/reducing ratio when mixing your material, and allow 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.
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 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.