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 effect 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. Globing 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.
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 in sheets. Though all paint problems are aggravating, there's nothing worse than leaving the driveway with a shiny new paintjob 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 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 within the recommended flash times for the material you're using.
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" truck 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 wash the area with a solvent (thinner, or reducer) wetted rag and then clean and respray 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 respray. 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.
If you don't get runs you are not trying hard enough. Also sometimes known as sags, hangers, or curtains. Runs are, along with dust, one of the most prolific paint problems for the hobbyist or occasional painter. The most common causes are holding the gun too close to the surface, moving too slowly across a panel, and double coating an area. Over-thinning/reducing is also a possible cause, along with trying to paint in an environment that is too cold. To fight runs and sags, you've gotta hold your gun perpendicular to the surface and keep it a steady and correct distance from the panel, all the while moving it fast enough that you don't pile the paint on, yet slow enough to get good coverage and flow-a process that comes with practice and experience. If you do 'hang some curtains," in some cases you can wipe the area with a solvent-wetted rag and then clean and respray 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 respray.
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 course 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 course 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). Also use 1,200-grit or finer paper for color sanding.