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—and 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 products 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 within the recommended flash times for the material you’re using.
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 makin’ 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 Bandaid.
Also sometimes known as runs, hangers, or curtains—sags are, along with dust, one of the most prolific paint problems for the hobbyist or occasional painter. I have to admit, in my many quests for slick finishes I’ve created way more than my share of curtains in my day. 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 (I’ve done that one too). To fight runs and sags you’ve gotta hold your gun perpendicular to the surface and keep your gun a steady and correct distance from the panel, all the while moving your gun 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 sew 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.